Chipmunks. Those things can get into about any space that a mouse can get into, and they drag in nuts and leave the shells. Got a couple into the crawl space portion of the house after a HVAC repairman left open a hole when working on the air conditioning lines. Chipmunk ate all the insulation around a CATV line that went through that hole also. I sprayed foam insulation in the hole to block it, they thought that tasted good too. Had to evict them again, and used some of that moldable metal that is used on ducts to seal the hole. That worked.
Those critters are destructive.