Quiet As A What?

So, we've discovered we have a mouse living on the second floor of our house.

Our first hint was odd sounds in the middle of the night.  The first time we heard the noise–a very faint pit-pat-pit-pat–I made a 3 a.m. tour of the house with a baseball bat in hand, and when I saw the dog was sound asleep on the first floor (since developing hip dysplasia, Abbey has rightly determined, to her disappointment, that she can't make it up the stairs), I decided that we had likely opened up our house...