A rewrite of detection and perception rules (first draft)

[A Butter Idea]

Intent
The current rules as written lack some flexibility when it comes to ruling on characters with unusual senses. For example, a hero based on Matt Murdock (a.k.a. Daredevil) would be permanently blinded but possess hearing as a precise sense. It's up to the GM to do the heavy lifting and reinterpret the rules in a way that makes sense for this particular character.
My rewrite of the rules is an attempt to elaborate on current mechanics as they pertain to stealth and perception, making them both more robust and sense-agnostic. In doing so, it opens the door for additional character concepts. I plan to incorporate any feedback into future drafts; I want to write these rules in a manner that is at once clear, concise, and fulfills the goals I outline above.

Note: "Subject" is a catch-all term for any creature, object, or individually-targetable entity.

Detection conditions
* Observing: You know a subject's exact location. You can directly target a subject you observe. You can only observe subjects using precise senses. As long as you observe a subject, you also detect and notice it.
* Detecting: You know the space that a subject is in (within a 5-foot square). If you are not observing the subject, you are off-guard to it. You can directly target the subject, if you are not observing it, you must succeed at a DC 11 flat check or you fail to affect it. You can detect subjects using precise or imprecise senses, but not vague senses. As long as you detect a subject, you also notice it.
* Noticing: You know that a subject is or recently was present. If you are not observing the subject, you are off-guard to it. If you are not detecting the subject, you cannot directly target it. You can notice subjects using any senses, and subjects remain noticed until you can no longer reasonably believe that they are present.

Note: Some abilities only work against targets that don't notice the user.

Targeting spaces and areas
If you notice but can't detect a subject, you can guess at its location and target where it might be. When you use an attack, spell, or ability that targets a subject, you can select a space (a 5-foot square) as a target instead. The GM rolls a DC 11 flat check, as well as the check needed for the ability to affect its intended target (usually an attack roll or saving throw), both in secret. If the space is not occupied, the flat check fails, or everything in the space would be unaffected, the GM only reveals that nothing was affected.
Area effects do not directly target subjects, and so detection and sense-based conditions do not invoke a flat check. Furthermore, for each subject you notice but don't detect, the GM secretly rolls the check for that subject even if it isn't in the effect's area; if the subject is unaffected, the GM doesn't reveal whether it was due to the check result or the subject's location.

Concealment
When you're in thick fog, underbrush, or some other obscuring feature, you have concealment, making you harder to target. Concealment is relative, so you might simultaneously be concealed from one creature and not another. Concealment obscures against one or more senses.
When a creature targets you with an attack, spell, or other effect while you have concealment against all of its precise senses, it must succeed at a DC 5 flat check or it fails to affect you. Both lesser concealment and standard concealment provide this benefit. In addition, while you have standard concealment against one or more creatures' precise senses, you can Hide to stop them from observing you.
Usually, the GM can quickly decide whether your target has concealment. If you're uncertain or need to be more precise, draw a line from the center of your space to the center of the target's space. If that line passes through any terrain, object, or creature that would obscure line of sight/sound/scent, the target has concealment against vision/hearing/scent.

Cover
Mostly unchanged from RAW.
Typically, cover prevents subjects from being observed by all senses, but only prevents subjects from being detected via sight. Most senses other than sight can be used to detect subjects behind cover, so long as an unobstructed path no longer than the sense's usual range exists between the sensing creature and the subject; this path can travel through the air (with the exception of tremorsense, which travels through the ground).

Sense-based conditions
* Unnoticeable: This condition always lists one or more senses. Creatures cannot observe, detect, or notice you using the listed senses. If a creature was observing you using any of the listed senses, it no longer is. The effects of this condition take precedence over other creatures' checks to Seek you.
** Invisible: You are unnoticeable by vision.
** Silent: You are unnoticeable by hearing.
* Overstimulated: One or more of your senses are overwhelmed or obscured. This condition always lists the senses affected. All subjects have concealment from your overstimulated senses.
** Dazzled: Your vision is overstimulated.
** Ringing: Your hearing is overstimulated.
* Impaired: This condition always lists one or more senses. All subjects are unnoticeable by your impaired senses. If a Perception check requires you to use an impaired sense, or does not permit the use of any senses which are not impaired, you automatically critically fail the check. You take a –2 status penalty to Perception checks involving an impaired sense. If all your precise senses are impaired, the status penalty is –4, and all normal terrain is difficult terrain for you. Perception checks for initiative typically involve vision and hearing.
** Blinded: Your vision is impaired. Blinded overrides dazzled.
** Deafened: Your hearing is impaired. If you perform an action that has the auditory trait, you must succeed at a DC 5 flat check or the action is lost; attempt the check after spending the action but before any effects are applied. Deafened overrides ringing.

Examples of abilities and effects with updated descriptions

Hide (1-action)
You use your surroundings to prevent other creatures from observing you. The GM rolls your Stealth check in secret and compares the result to the Perception DC of each creature observing you but that you have cover, greater cover, or concealment from. You get a +2 circumstance bonus to your check if you have standard cover (or +4 from greater cover).
Success: If your source of cover or concealment obstructs all of the creature's precise senses, it cannot observe you.
If you successfully avoid a creature's observation but then cease to have cover, greater cover, or concealment from it, it resumes observing you. The creature can observe you if you do anything except Hide, Sneak, or Step. If you attempt to Strike the creature, it remains off-guard against the attack, and then begins observing you. If you do anything else, the creature observes you just before you act unless the GM determines otherwise. The GM might allow you to perform a particularly unobtrusive action without being noticed, possibly requiring another Stealth check.
If a creature uses Seek to observe you, you must successfully Hide to prevent it from observing you again.

Seek (1-action)
(First paragraph unchanged from RAW.)
The GM attempts a single secret Perception check for you and compares the result to the Stealth DCs of creatures you are not observing, or the DC to detect each object in the area you are not observing (as determined by the GM or by someone Concealing the Object). If you use only imprecise and vague senses to Seek, you cannot get a result better than Success; if you use only vague senses to Seek, you cannot get a result better than Failure. The unnoticeable condition takes precedence over the results of your check.
Critical Success: You observe any subject you critically succeed against.
Success: For each creature you succeed against, you detect it if you weren't detecting it before, and you observe it if you were detecting but not observing it before. You also observe one object that you weren't previously observing.
Failure: For each creature you fail (but don't critically fail) against, you notice it if you weren't noticing it before. You get a clue as to the location of one object you are not observing.

Crystalline Dust (2-action) Feat 5
You've learned to disperse the crystalline motes coating your body as a haze. You gain lesser visual concealment from all creatures for a number of rounds equal to half your level.

Mist (3-action) Spell 2
You call forth a cloud of mist. The area in the cloud provides visual concealment. You can Dismiss the cloud.

[Loreguard]

At a glance it seems pretty reasonable.

I think this sort of thing also would become even more relevant for StarFinder, where Alien species should be able to rely on alternate senses more often that we might otherwise see in the other more traditional low-tech fantasy stories.

So I'd go so far as saying it would be wonderful to see Starfinder rewrite the rules to something akin to this, and people could easily re-port Pathfinder to use the more up-to-date rules if they are helpful.

[Megistone]

I like it. The only part that I think isn't immediately clear is the one about standard and lesser concealment.

[A Butter Idea]

For my second draft, I'm considering grouping cover, concealment, observing, detecting, and noticing as Circumstances; pseudo-conditions which are relative and usually provided by the environment.

[Teridax]

I like this approach and have thought about the issues behind the way detection and senses are so visual-centric in Pathfinder, though haven't come up with any kind of solution like this. I think the idea of setting detection-based conditions apart as these things automatically generated by environmental factors is a good idea, because targets becoming observed, concealed, and so on often happens automatically from other stuff that's happening, and not just from specific things a creature does.

For me, the issues with senses and detection become apparent when listing out all the mechanics that tie into them:

You have different conditions marking states of detection, from observed to unnoticed.

You have different senses, which each have different states from precise to vague (with an implicit fourth state for not having the sense at all).

You have different conditions determining how senses are impaired, which the OP simplifies using a common structure.

You have different conditions determining how detectable, rather than detected, something is. This overlaps heavily with detection conditions.

You have environmental factors that can provide various conditions, generally localized to the bit of the environment. These can be conditions that impair senses, conditions that affect how detectable someone is, or both (and often it's both). Mist in-game, for example, conceals both creatures inside from the outside and vice versa.

To me, this registers as extremely complicated and overwrought for what generally just amounts to asking "how well can I target this creature?" in encounters. It's not just that different senses matter only fairly situationally and aren't super well-handled when they do come into play, as pointed out in the OP, there's a lot of overlap to these mechanics that can sometimes make simple situations difficult to grok. The OP does a good job of addressing the bias towards sight, but I think there's potentially room to simplify things too, maybe even lean into the game's system of numeric bonuses and penalties instead of relying on conditions each time (or at least tying conditions to those modifiers).

[platonic-paperwork]

Oh I like this!

In the interest of using this to streamline the user experience a bit, like Teridax brought up, how about using the precision of senses as the names for the degrees of detection?

i.e. You can have either a precise, imprecise, or vague awareness of a subject. Precise awareness = observing, imprecise awareness = detecting, vague awareness = noticing.

Your degree of awareness of the subject is equal to that of the most precise sense you are detecting them with.

And the environment sets an upper limit for the precision of particular senses.
Maybe add a degree of sensory precision in between precise and imprecise, which gives lesser concealment, to round out the effects.

(Everything else works the same as above).

The end result is mechanically the same (same miss chance etc) but this would hopefully feel more like the player is proceeding directly from "how good is the best sense I'm perceiving them with" through "is there anything obstructing my senses" to "what is the end mechanical result", with each step being rooted in what is tangible to the character. No need to mentally convert from the imprecise sense to the detecting condition, just keep track of how good your current best sense is and go straight to the relevant impact.