I manage a large apartment complex (chickens are at moms) The last phase of the complex was supposed to include the office....it was never built.
I HAVE to keep my door locked at all times even though it is a door that matches all the rest of the doors and says PRIVATE RESIDENCE or people walk right on it. Now you would think the fact they walk in to my kitchen or living room might give them the clue that this is not the office............but having found a couple hanging out in my living room when I was in the shower (you can hear the water from the living room) I know I HAVE to keep that door locked at all times.
Ps.....if you have a relative that has an emergency key and thinks it is an open invite to come on in, change the locks. Make up a good reason and forget to make them a key copy...............I had to do this with my own Mom, I no longer enjoy random people dropping by (see above) but Mom had a key and I was out of town, so she brought her friend over for a tour of MY place. The friend liked a bird house and was going to take it home with her! Thankfully, the decided maybe they should ask first, I said NO you can not have the bird house my son made in scouts!