Mice in the walls - what to do?

We can hear them (or something ) going up and down inside our dry-lined walls. Also under our only timber floor (on battens over a concrete slab). Yet they don't appear in the living areas of the house. No droppings anywhere. No physical evidence. Just the scampering and the chewing sounds....

What do we do?

Mice can often get in to an attic and from there down the walls but may not find an opening into the house. They can do some damage by eating away at stuff and anyway it its not particularly pleasant to have to listen to them. They can be alarmingly loud esp at night. We had this problem a couple of years back and got rid of them through a combination of traps in the attic and also we opened the hole around the pipe for the radiator in the room where we could hear them most under the floor boards, put a box ( Mr V made one specially and fitted it around the pipe ) above the hole and put in some traps so they could come out into the box but go no further. But you could just seal that room off from the rest of the house if there werent too many things in there for the mice to hide in.

There are some previous threads here which make reference to this problem.

Thanks for those. Will look up the threads.