Little rugrat brought me a mouse tonight. And by tonight I mean 2 am. It was alive and well too! Spent the better part of an hour chasing that little sucker all around the house. My cat was enjoying the chase, but otherwise not actually contributing. Finally cornered it and sadly killed it. Would have preferred to trap and release it back outside, but it just wasn't going to happen. Besides, my cat would have inevitably caught it and brought it back in anyway.
I'm assuming it came from outside, but I think I'll spend tomorrow scouring the house - I don't want to deal with a nest. I live in the country, this is just something I have to deal with.