I would get some "Sorexa CD" from your nearest agricultural store, its a professional use product, I always used it when I was a professional pest controller. put lots of little trays/saucers down all over the areas where the mouse has been seen, they are sporadic feeders, one or two trays of poison wont work. "DO NOT" block up any holes, if you close any holes the mouse/mice may not be able to get to the poison and it may move to another area of the house. after the problem has gone, then you can block up holes. It could easily be just one mouse, could just as easily be more than one. Good luck.