Your unit may be causing a certain amount of air to be drawn OUT of the window/AC unit. By doing this it needs to "make up" air from somewhere else. The make up air could be coming from a supply air duct or return where a mouse has croaked. Or if you don't have air ducts then from a void in the exterior wall where mouse has croaked. This is a wild hair guess but the only thing I could think of. You would be able to repeat this by putting a fan blowing out the window with all doors and windows closed. Then sniff around cracks/ ducts / returns until you smell the stronger source. You could also try this in another room and maybe duplicate the results. If it is in the ducts/return then it could be anywhere in the house and you probably need a duct cleaning company, at least check your furnace air filter for slime.