First thing I would do is go around and seal up any spots where it can get out of the house - under doors etc. Then I would shut each door in the house and search room to room.
try spreading some talcum powder or flour around the floor to see if its in the room. Also as staited earlier put a heat source out and a mice or what ever it eats waite till late at night when everything is quite with a torch you might find it out
In my experience i have always found my escapees in a few hours of searching curled up under their heat light in the enclosure or trying to get in with their mates in other enclosures...