tomBitonti
Hero
I rule that when PCs are expecting trouble (which is most of the time, hence the default situation, when adventuring) they can use passive Perception to detect threats. If they're distracted (eg navigating, mapping, tracking, foraging, deep in conversation, or hustling a captive) then their passive Perception has disadvantage.
NPCs who are expecting or looking for trouble (raiding parties, guards on duty, border patrols, beasts of prey on the hunt) likewise use passive Perception. When they are in places they deem safe (at home, at the market in their home town, in their own territory) their passive Perception has disadvantage.
I contest passive Perception with the Stealth checks of hiding creatures. If a creature isn't hiding, and the perceiver is not distracted, then it is automatically detected; if the perceiver is distracted, I use a special form of passive Stealth check which is the creature's bare Stealth modifier with no additions (so normally in the range from -2 (zombie) to +11 (assassin)).
For objects, any object in plain view is automatically detected. Objects that are concealed or disguised can be detected by a PC if his or her passive Perception matches or beats the DC to notice the object. Every point by which you beat the DC increases the distance from which you notice the object by 5 feet. For example, if Muhrek's passive Perception matches the DC to notice the secret door, he spots it when he is in the space adjacent to it. If he beats the DC by 1, then he spots it from the next furthest space.
I'm at great pains to set consistent DCs throughout a campaign. As characters improve, so should their chances of spotting things. So if a lock is Very good, its DC remains 25, whether the PCs are at level 1 or level 15. Having exploding d20s still means there's always a chance of success or failure, whatever the level, and I also enable players to roll dice while obscuring the certainty of outcomes, which helps with immersion.
This all sounds good.
Then, "any object in plain view is automatically detected" becomes "any object *not* in plain view is *not* automatically detected", with a roll required for such an object to be detected.
Mapping that to the normal senses (sight, sound, and smell), which can be augmented depending on special abilities (say, a spider being able to sense by vibration objects which are moving in their web), "plainly visible" extends as "making an obvious noise". In the case of smell, a scent type ability typically automatically detects objects, or at least creatures, in a set radius, but I would extend that to creatures with normal senses for an object or creature which is exceptionally smelly.
For an object or creature which is invisible, and absent a feature which otherwise is obvious (is making a lot of noise, or is especially smelly), the the consequence seems to be that the object or creature is no longer automatically detected. A roll is required without the object or creature making a hiding check. And if the creature went invisible then hid, the difficulty of the roll should be increased.
Thx!
TomB