We got it from a reptile store that sells feeder rodents. They sometimes leave thier back door open when its warm and the mouse probably came in from outside and they probably thought it was one of their feeders escaped.

Since it was a wild mouse, I don't want to feed it to my snakes, regardless of it having lived with us for a few months with no signs of illness or parasites. I just make it my rule that my animals only eat captive bred feeders.