I keep an old birdhouse on the hanger floor with a supply of 'mouse poison' inside. In my recent annual the A&P did find evidence of mice inside the aircraft from it's previous life, old evidence. Another trick I've heard is to have a supply of toilet tissue boxed in the corner to give a visual of mice present, they like to chew the stuff. A 'bait station' from a home supply store would work too, tucked away in the hanger corner. I detest mice.