Being careful and being silly are too different things. Most people don't lock the door to the bathroom when their home.
Most people's toilets aren't mimics, either. In a place that's known to be deadly, it would only make sense to be extra-cautious. I don't always check to see if my ingition is wired to a car bomb, but you can bet I'd check EVERY time if I knew some mobster was likely to put a hit on me.
Honestly I don't really have that much of an issue with detect magic as an at will ability. In many ways it's less annoying to a GM than the paladin's constant detect evil ability used to be.
Against invisible creatures it would be able to detect the presence of an aura the 1st round, and possibly be able to determine that there is an active illusion in a specific 5' square on round 3. However that does not mean that the invisible creature is silhouetted by some glowing aura that negates invisibility. Instead the wizard is aware that an weak illusion emanation exists in that general vicinity. It could be a permanent ghost sound, a magic mouth, a trap or an invisible creature.
You and your allies would still be easier for the invisible foe to hit and would still be flat-footed against attacks unless you had blind-fight. However you might be aware that you needed to take some countermeasure such as raise a see invisible spell or a dispel magic.
By the same token a magical trap that uses an alarm trigger would show up as a weak abjuration emanation in a 20' radius. This emanation could be an alarm spell, an arcane lock, or an obscured object. I personally would rule that spell traps are akin to spellcaster spells, they don't show up as auras until they are actively triggered by the alarm spell. A spell trap that triggers a dispel magic effect wouldn't be that different than a spell trap that unleashes a destruction effect.
Also keep in mind that the mage is going to have to be up front to do his detect magic trick as he doesn't want the personal magic of the other PCs to interfere with his ability to detect magic especially if the PCs have big magic active and the aura in the room is weak. A mage up in front of the party is definitely more at risk than one that is huddled behind the fighter and cleric.
I personally would rule that spell traps are akin to spellcaster spells, they don't show up as auras until they are actively triggered by the alarm spell. A spell trap that triggers a dispel magic effect wouldn't be that different than a spell trap that unleashes a destruction effect.
I understand the practical reason for ruling it this way, but by RAW I don't see why an Alarm spell is detectable and a Spelltrap isn't.
Also keep in mind that the mage is going to have to be up front to do his detect magic trick as he doesn't want the personal magic of the other PCs to interfere with his ability to detect magic especially if the PCs have big magic active and the aura in the room is weak. A mage up in front of the party is definitely more at risk than one that is huddled behind the fighter and cleric.
I don't know the mechanics for determing what number of auras and where is too many for Detect Magic. The spell doesn't state any, it just makes a vague reference that auras may obscure other auras, without really elaborating.
If someone has a system worked out for determining when auras obscure Detect Magic, and when they don't, I would like to see it.
Since Pathfinder take origin from D&D 3.5 Edition I think we can refer to D&D FAQ v.3.5 78 Update Version: 12/21/07
Is it possible for detect magic to locate an invisible
creature?
Yes, although not very efficiently. Remember that detect
magic reveals the location of magical auras over the course of 3
rounds. A creature rendered invisible by a spell or magical
effect could be located via detect magic, but only after 3 rounds
of concentration. Furthermore, the invisible creature must
remain within the spell’s area for the entire 3 rounds of
concentration; if the creature moves out of the area, the process
must start again from the beginning. However, even if
everything works according to plan, you still don’t necessarily
know that you’ve found an invisible creature—at best, the
caster of detect magic would know that she had located a faint
aura of illusion magic in a particular space.
Can you use detect magic to detect supernatural effects?
For example, can it detect a wildshaped druid?
Supernatural abilities are magical, and thus their effects
would produce magical auras. Although the detect magic spell
doesn’t have a line entry for supernatural effect, you can use
the same line as “Magic item (caster level)”—a supernatural
effect’s caster level is equal to the creature’s Hit Dice unless
noted otherwise (Monster Manual, page 315).
Can detect magic be used to identify magical traps?
Would nondetection block this?
Detect magic locates magical traps, but it can’t identify
anything more than the location and strength of the aura
revealed (and, with a successful Spellcraft check, the school of
magic involved). Thus, technically the spell doesn’t actually
reveal the fact that the magical trap is, in fact, a trap.
Nondetection wards a trapped object from detect magic
(requiring a caster level check to pierce the nondetection). An
even better option is Nystul’s magic aura, which can conceal a
magic trap’s aura (or change it to appear as that of another spell
of your choice).