I love that they made the mouse holes available to us as I added one to my run-down orphanage. Lots of mice spawn there! You should be able to see the hole in the wall near where the mice are scurrying, and if you watch them for a while you will see them return to the hole. I know on my home Lot my sims can set mouse traps if I click on the hole, not sure if you can do that in community Lots. But the mice do return after a while anyway and then they gradually multiply.
You can just go into build mode and delete it.
https://dl.dropbox.com/s/46ivi6g6un84fgv/Mouse%20Hole.png?dl=0
It's nothing to do with the Lot trait, as EA did not release that trait for our usage so you won't find that anywhere other than the CL Lots EA already added the trait to, unless it has been modded in.