I've heard many stories about lost snakes popping up weeks, and sometimes even months later.

Keep looking. Don't limit yourself to "warm" places. They will often hide someplace cool if it feels secure. Whenever you have a chance to be quiet and still...listen. If you can just sit in the evening with a book or something equally silent to occupy yourself with, you may hear him rustling around. (I found a lost one that way once)

I had a friend with a rosy boa that got lost...was gone for two years. Then he moved to a new house. When he was unpacking boxes, he found the snake alive and well in one of the packed boxes! True, happy story!