I have a problem with the following rule taken from invisibility:
A creature can generally notice the presence of an active invisible creature within 30 feet with a DC 20 Perception check. The observer gains a hunch that “something's there” but can't see it or target it accurately with an attack.
Consider a stealthy spy, with +30 to stealth. When fully visible this spy is able to sneak with 30 feet or his targets without them ever noticing, but the second he becomes invisible you get a hunch someone is there and it suddenly gets much easier to detect him.
Should the DC not somehow depend on your stealth modifier, instead of just beeing a static number?
I would suggest the key word here is active. This is a guy not sneaking but simply walking around and doing stuff.
Thx for the quick answers, that makes sense.
If you are invisible, you gain a +40 bonus on Stealth checks if you are immobile, or a +20 bonus on Stealth checks if you're moving.
So your Stealth 30 thief would effectively have a 50 in the skill while invisible.
I always felt that the DC 20 to notice the presence of an invisible creature is that uneasy feeling you get that someone is watching you. We all get it from time to time. An unsuspicious person might look around, and seeing noone might disregard it. A more suspicious person might do a few tests to check for an invisible creature, if those fail they might go about their business. A person who has good reason to believe that an invisible foe may be present might search for until he finds something, or is sure that no invisible creature could possibly be present.
Not calling for the check if a creature is using stealth doesn't make sense. Stealth can be used untrained, and there are only three actions that you can't perform while using stealth: Attacking, Running, and Charging. Anything else is fair game. And when wouldn't you use stealth? Even while double moving you're likely to get a higher DC than 20. With only a few ranks in stealth, you're assured to get a higher DC.
The stealth DC is the DC to actually hear/smell/whatever the creature, not just get a "hunch"
This is just a house rule, I guess, but to avoid the paradox of an invisible creature actually being easier to notice in some cases, I won't make the "hunch" check unless the invisible creature does not have cover or concealment. In other words, he's using invisibility to hide out in the open.
I also make the "hunch" check if a creature, invisible or not, is within 5 feet. Even if the target makes the role, this likely won't spoil any surprise round, but it does prevent creatures from standing that close for any length of time.