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.