Was he at the in-laws house when he got out or at home? He is probably not far, and the best time to look for him will be very early morning or late evening when everything is very quiet. A scared cat may not come out even if it hears it's owners calling. He may be coming out at night wondering why you aren't letting him in.
You can try putting a box outside with some of his stuff in there and see if he goes in at night.
Good luck and keep us posted.