It's happened to me also.

* First, they usually don't go far. Hopefully that's the case with yours.
* Second, they can fit in places you never dreamed of so look in places that you are 'sure' they couldn't go.
* Third, start making some good hides for it. Take a small box, put it on a heat pad or somewhere warm against a wall on the floor. Do this with as many as you can. If you make the hides well, they'll eventually find one and like it enough to stay. Of course, keep the lights off & keep traffic in the area to a minimum.
* Fourth, you can also try dragging a F/T mouse around the corners of the room up to one of the hides & leave it in. That worked for me.

Good luck