Ahhh Mice. My neighbor growing up had them in her basement...we had woods behind us as well. My mom was always being asked to get one out! LOL. She was always saying "they need a warm place too."
Turned out...my neighbor had a hole near a basement window where they were getting in.
Next time you see one, try to follow it to see where it may be coming in. Seal up any holes you find...inside and out. You'll be suprised to find that they can squeeze through the tiniest of holes. When we were renting our apt last year I was there not even a week and I sat down to watch the news only to see an adorable (yet disturbing) little mouse pop up out of the floor! It was near my doorwall so I figured maybe the door needed to be sealed.
When I pulled back the carpet to see where the hole was I didn't find it in the corner. It was about four inches away from it! The hole was maybe an inch or so around. Somehow he made a hole right up through a CONCRETE floor then chewed through the wooden sub-floor! I suspect the foundation is crumbling. Anyway...neither maintenence nor myself could find a hole outside thus leaving me with the theory that he was burrowing under the building.
Maintenence sealed up the hole in the apartment, and my hubby caught the mouse in a coffee can and moved him away. Never saw a mouse after that.
Sorry for the long story...I just felt the need to share!
If sealing up holes doesn't work...you'll deffinately need to find a pest control company. They should be able to find any points of entry for you to seal up.
Hope this helps!