DrunkonDuty
he/him
In the case of someone on the other side of a wall, one gives a potential observer a hearing* based Per check. Simple really. The GM might decide the wall muffles sound enough to give a penalty to that check.
In the case of Invisibility and an otherwise clear line-of-sight I would also allow a hearing* based Per check at the normal rates (ie: no modifier for the invisibility) OR a sight based one that used the Invisibility modifiers. This sight based Per roll would basically be a chance to notice scuff marks on the floor, dust swirling in the air, that sort of thing.
* or smell in the right circumstances.
In the case of Invisibility and an otherwise clear line-of-sight I would also allow a hearing* based Per check at the normal rates (ie: no modifier for the invisibility) OR a sight based one that used the Invisibility modifiers. This sight based Per roll would basically be a chance to notice scuff marks on the floor, dust swirling in the air, that sort of thing.
* or smell in the right circumstances.
See what I did there?) Therefore, if the guard hears odd noises from a seemingly empty space they should get very suspicious and at the least give the space some very fine scrutiny (look for foot prints in the dust et.al.) Yeah I'd give penalties to Per rolls for noisy rooms, high winds, whatever. But the guard still gets a roll. As an aside it occurs to me that this would make guard dogs very useful: Scent and superior hearing for the win.