That's the trouble with mice, they can get into very tiny spaces. So as long as they can climb up something they'll get in those holes. They're about 1" square.

You should also check to see if the rubber plugs are in the big holes behind the kick panels. These holes are about 1-1/2" round and are to allow access to the bolts on the trailing center edge of the front fenders. I found my car was missing both of them. You have to pull the kick panels to see if they are in.