If the tank is not in a room that needs to be dark ever keep the lights on. If he didn't move out of that room he'll be more likely to stay put because of their night time enjoyment. Of course keeping a mouse in a tank close to a blanket or something to hide in is a good way. I've also read that putting power of some sort accross door ways or along walls will help you narrow down where the little guy is. They def like to move along the sides of wall rather then the middle of the floor. But don't give up. Alot of found snakes don't turn up until a few weeks or even months later. it's winter so he'll prolly stay around for heat, but he may not move as much either, expecialy if there arn't mice running around in your house.