Sounds like the problem i was having...what i did was started looking for where the biggest concentration of them were...then when i would see one i would chase it to see what way it came in. Most of the time i noticed they were coming from a radiator in the kitchen. I then steel wooled and trapped the hell out of the radiators perimeter needless to say if a mouse got through the wool it would fall onto a sticky trap where it would then meet a gruesome death depending on my mood...needless to say the mouse fiasco has died down ALOT.

With all that said...i suggest finding there source of entry