Put a humane trap near where you found the "droppings". Mice can live anywhere. They come out at night and travel along the floor boards. I had a mouse problem one time. They were living in the walls of my bathroom, under my stove. I also had ant traps around my apartment. One night I heard this weird noise, like something rolling across my hardwood floors. I got up with a flashlight. I caught one of the mice playing with my ant traps!!!