Well, that microchip will, at least, prevent any major laboratory from paying $50.00 bucks for the animal and taking him into inventory for their next cancer experiments! I agree with you; he probably was stolen; and, more than likely, by someone who's fairly local, too. Watch the, 'Pet's For Sale' columns of local newspapers and periodicals like Graigslist; your, 'lost' pet might turn up there.
Your next best move is to notify all of the animal shelters within 50 miles of your home. Send them a flyer with a picture (if you've got one). It's a good idea to phone local animal shelters once a week, too. Remind them that you're still looking, there's a reward, and ask for any advice they might have. A reward of, about, $200.00 dollars, with a, 'no questions asked' clause should do it.
When my sister's dog ran away she put up flyers all over her lake community looking for the animal. Someone found it for her, too. That little poodle had traveled more than 4 miles over the local roads, and was found all the way over on the other side of the lake! Good luck! I'm a retired dog breeder, myself. So you've got my sympathy.
Oh, by the way, you can prevent something like this from happening again by installing a, 'driveway alarm' along the sides and/or rear of your home. (UPS hasn't snuck up on us in years!)