I'll try to keep this short and sweet:
I live on the first floor of a 3-family home. My landlady, who is getting on in years, lives on the second floor. This past autumn, we had an issue with mice. My landlady called an exterminator to take care of the problem. Mind you, the entire neighborhood has had a problem because we've got several restaurants within a two block-area (better than six legged creatures, I say). Now
I haven't seen signs of mice since the one my dear kitty so proudly deposited on my bed (dead, thank goodness) back in October or November. I'm not convinced that we still have them. The exterminator is not convinced that we still have them. The landlady has been adamant that they're there. She insists that between the time she makes her coffee in the morning and the time she goes to clean up from having made breakfast, there are droppings on her kitchen counter. Finally, she saved some of the droppings for the exterminator so she could prove her case.
Turns out the "mice" are pooping coffee grounds.