There are a lot of mice around at certain times of the year. They come from the outside, so that's where to begin. New construction often causes problems in newly-developed areas. Mice are driven mostly by curiosity, and will enter any hole or crack they find in a structure. If they find it better on the inside, than it is on the outside, you have mice.
Mice can live in a freezer, below freezing all of their lives, feeding on nothing but frozen foods.
Naturally, we don't mean YOUR freezer, the one you might have at home, but in large, commercial operations, this happens quite often. It is imperative that you do this. A mouse can fit in a hole the size of a dime. If your little finger goes in there, the mouse can. Once inside, mice can get anywhere.