The only thing I can think of if he is in your walls is that once fall or winter roll around he might come out seeking a heat source. Depending on if it gets cold where you live.
But don't give up hope! And remember, just because he wasn't in a spot last time you looked doesn't me he won't be tomorrow (Although I'm sure you have been re-checking everywhere)
Bitter cold Appalachian winters.
I've totally, literally exhausted myself rechecking every thing, every day.