It happens. If you have hardwood floors you can put down flour everywhere and that way if he moves in the middle of the night you'll have somewhere to at least begin to look. Check everywhere. Anywhere you would expect him not to be, check there. You would expect them to go to a heat source, but any time I've had a snake escape, they've always ended up in a relatively cold area. You could also along with leaving food out for him, leave one of his hides out on the floor near the rat. Good luck on finding him.