It's not THAT unusual, on entering a house through a locked door, to find your way blocked by another door that needs unlocking. (One or both of these doors might even have two locks.) And then inside this locked house there will quite probably be rooms with locks on their own doors. And inside some of those rooms, cupboards, with doors, with locks on 'em! And look inside enough of these lockable cupboards, and you might well find a safe. With a door with a lock on it...