We have now found two dead mice--probably, but not certainly, killed by our two cats about 6 weeks apart. We live on the third floor of an old apartment building, with an enclosed front staircase, which is how everyone accesses their front doors. Our front door is warped so that there's a pretty big gap between the bottom and the jamb, and the weather has been getting colder (the time of year when mice generally come inside, I've heard). Both mice we found far from the kitchen, near the front door (in the foyer, actually). We searched the kitchen and closets and found no signs of them--no droppings, no chewed boxes or fabrics, and no holes. Could they be coming in from outside? We covered the gaps in the door and put traps in the front hall closets. Could they be coming in from outside or are they definitely living in the walls, perhaps mostly kept at bay by the cats?
Side note: I've never heard a neighbor complain about mice.