I assume you shut the door every night, so I also assume this is a chain conveyor-type unloader and not the auger style.

On the chain conveyor style, there is an  "gate adjustment" hole = roughly 1 1/2" - 2" pipe cap, that should be covering that hole, on left side, back by big aluminum door(left side, as if you were standing in front of unloader).
I would guess, if this was left open , mice could easy get in there and build a nest daily.