?? Check absolutely everything. What kind of a pen? You didn't leave the gate open did you? Anything he could of pushed through?

To get him back...take a large bucket or something similar and put a rock or something heavy in it that cant get knocked over. Try to make a ramp or something, and put some food that smells yummy in there. As long as he's in the house still...he should manage to wind up in the bucket.

That is awfully strange.