I agree. Can't abide thieves. This contractor should do what's right and return this firearm to the person who stashed it behind that wall. He should go to the county courthouse and look for the property deed, then the local library to check the obituaries, then find the closest living relative to the former homeowner and return this firearm to them that they had no clue they even owned. Check Find A Grave or Ancestry dot com. This person obviously thought a lot of this weapon or they wouldn't have gone though the trouble to keep it in such a safe place. It's important that this family heirloom be returned to it's rightful owner instead of the current homeowner who has obviously attempted to swindle a free gun in the wall when they bought the home.

After exhausting all efforts to locate either the original owner or closest living relative, I would then invoke the law of "Finder's Keepers, Losers Weepers".