How can you get rid of mice that are behind your walls?

For months I've been hearing things happening above my ceiling and then it progressed to the floor level of the walls. It sounds like the tering of mice. I've never once seen a mouse in the Apartment (and its a small place here with no furniture other than a chair a computer desk and a bookshelf- That's it. No bed, no tables, no couch- basically no place for them to hide. I think I would've seen one if they were in the apt already. There are also no holes that I can see in the walls. I believe they may be trying to go through a dig into the apartment. How can I get rid of them while they're still in the walls?