Personally I feel on my own little homebrew, listen search and spot, taste analysis, smell all the senses are static abilities based on a race. ( I use a %skill based classless system, but it can be changed to a static d20 number rather than percentage though i tend to find that with d20 the 'heroic' difficulty was far too low to what was acheiivable by level 10 making later adventuring post 10 boring dull and easy)
Without going too much into it each character has: Perception/Awareness of between 70-85% of which whenever the character wants to spend their turn listening, searching, looking, smelling, tasting to identify something or somewhat they roll this sense score. Modifiers are applied to this based on the difficulty of what they are wanting to detect, as well as additional modifiers based on distractions (ie trying to hear a conversation through a window during a storm - light obstruction -10% - Heavy rain+thunder -40%. Other modifiers may include, target conversation is in hushed tones. Its so flexible based on a real scenario.
The base score of 70-85% is halfed (to 35-43%)if its a passive action. I.e the characters arent actively listening for something in particular, theyre just playing cards, while a guard stands watch (actively using perception for full bonus).
It was based on the premise that our senses are static, they dont improve, they dont decline (unless through accident or age). Even the blind or deaf dont actually gain better hearing or sight because they lose a different sense, they just rely on it more so they notice more though this could be argued on how a spot search or listen skill would improve, but hey going from 1 rank to 20 is a massive difference!)
Mainly these senses are static and in the hands of the GM.
A 'passive' halfed base percentage never contests another skill. But an 'active' one does. For example:
A party of Adventurers camp up for the night, make their stew and huddle around the camp fire, Hank takes first watch and patrols around the camp keeping watch and listeing out for intruders. Hanks Perception is 'Active' at 75% (human 75% -10% of Eric's snoring and +10% from being within the illumination range of the fire)
Presto is fumbling around with his hat and testing out some minor tricks. Sheila and Diana are talking about boys, while Eric is fast asleep snoring his head off while Bobby feeds Uni. Everyones senses are 'passive' as their attentions are elsewhere spending the round doing something else, while Hanks 'active', devoting 100% attention to guard duty.
Bob the antagonist thief notices the fire with a 'passive' perception check of 35%, +20% from illumination in a dark wood, +10% to the snoring of Eric, their is no additional modifier for conversation of Sheila and Diana as only the largest factor in each of the sense aids in any way (Eric's snoring is far louder than Sheila and Diana's chit chat and presto's muttering of incantations). rolls 40% its under 65% so hes noticed the camp.
Bob rolls his stealth check! rolls 84% Yikes its a fail aginst his 62% skill even with the +20% from cover of darkness!
The Gm rolls Hanks Perception of 75% and gets 68%! Hes heard something, but the stealth roll wasnt a fumble, so Hank knows somethings making some noise but not what it is, Could be an animal or something.
The Others with Passive percetion dont get to roll unless it was a fumble, Its all down to Hank.
Bob starts moving again fearing he'll be detected and rolls a total of 42% a Succeeding his skill chance by 40%
Hank invetigates using perception rolling his 75% skill but now with the additional penalty of Bob's skill success of 40% bringing his skill to 35%. Hank rolls a 46% and fails to notice Bob. Hank not knowing about the modifier is confident that it was nothing and continues to keep watch.
Bob continues for one more around as he begins to rifle through someones pack. His Stealth roll of 00% oh no a fumble! Even passive listerners get to roll!
The Others roll their passive perception rolls, Eric is asleep so he needs to crit this to wake up (10% of total skill) passive of 35% -10% from current noise hes already sleeping though for a total of 20%, he rolls a 99, a fumble, he farts, rolls over (but not into the fire, GM isnt cruel) but the Gm applies a further -10% check to everyone else as theyre now distracted by Erics smelly fart.
Sheila, Bobby, Diana fail to succeed but Uni's heitened sense detect and she bleats out a warning!
Hank rolls again this time unhindered by a stealth skill and succeeds, spotting Bob and raises the alarm!
(ok this system doesnt really work as well on a skill ranked system of DnD/PF due to the obscene difference of levels which is why I started rewriting my whole system out to RQ SRD when it went OGL)