We have a mouse in the ceiling. We can hear it skittering around and gnawing on things above the kitchen. In an effort to get rid of it, we had to get up into the ceiling and put a trap in the area. While doing so, we realized that a grille in the upper wall of our living room was actually the air intake for the (long since removed and not replaced) gas furnace, and it was effectively a nice vent for sending all of the heat in the living room up the furnace's roof vent. No wonder it's been so hard to heat the living room!
Lisa cut a piece of foam insulation from the scraps we have in the garage down to the correct size and plugged the hole. We'll find out in a few weeks how effective it is.
I hope the trap catches that mouse. It's a humane type trap, and if I hear clanking instead of chewing coming out of the kitchen ceiling, I'll know I need to go get the trap and take the mouse somewhere else and let it loose in a field somewhere.