it might be worth taking a broom-stick and going around the exterior of the house, right where the wood meets the foundation (concrete slab or block) and where the foundation meets the ground. take the stick and run it along that edge, and if the broom handle pokes through, that's a large enough hole for the mice to get in.
You'll want to plug the hole with steel wool REALLY tight, then follow with spray foam to seal the cracks.
that might leave some mice inside the house, but...it is what it is at that point and do what you must to capture them. mouse-poop is very dangerous, so please wear a mask whenever you come in contact with it, and change the vaccum bag IMMEDIATELY afterwards, or you are just blowing air over mouse-poop and spreading out the back end of the vacuum cleaner.
i've lived in "the country" my whole life and have dealt with mice in every home I have ever lived in. it happens. LOL
