Mice get in this time of year. A single mouse isn't an infestation, though it could turn into one if it is pregnant. Set a trap or series of them where the mouse travels.

I have had the occasional mouse get in my house when it gets cold.

Large holes should be repaired. Smaller holes/around pipes, stuff steel wool and caulk or spray foam.

Mice can get in through even a small opening around a pipe under sinks and behind laundry appliances.

I get it, I hate mice. They get into food and cabinets. Nobody wants that.