A squirrel can be found hiding on The Forum in a deep pit behind a door.
Get to the 3rd checkpoint. Reverse off of the ledge behind you and you'll find the door on the ground below. The squirrel will burst out almost immediately, so drive forward to avoid being hit by the swinging door.
It's a squirrel :)
This section is reserved for Confirmation/comments about this investigation. Investigations will remain open until advised by Redlynx that the final solution for each secret is found.