Master keys work for someone who needs access to a lot of properties without the hassle of carrying a whole ring of keys e.g in my rentals, but no one, even the maintenance people get one. If a homeowner is gone the Gen'l contractor can remove the lock and get it rekeyed to match your key and you would not know it was changed. I've also used a separate lock during a construction phase and then replaced it when the new tenant moves in. or a lock box with a 4 digit code I give to the subs. that code can change as well. if anything suspicious is going on. Also in most home with security systems there is a separate code which contractors can use to disarm the system. But it is best to change it entirely