Maybe they moved, and the dog jumped out some how and ran back to him home. Maybe it would be best for you (if you can) take care of him for a while and see if the owner comes looking for him (u live right next door right???) and if they don't maybe keep him or give him to someone...I dont think it would be fair to just leave him, I've seen a few times where people just don't want their dogs so they leave them to die..it's so sad :[ I hope everything works out good for you can chab