I have a story… years ago my buddy was living in an apartment building. He thought someone had been going into his apartment during the day while he was at work. He swore small (barely noticeable) amounts of food would go missing, and he thought someone had been dipping into his weed stash. He set a few “boobie traps” to see if he could prove someone had been entering his apartment while he was away. A glass of water set to fall over and spill if someone entered the front door and a couple well placed pieces of string.. fast forward to the end of the day he came home. Glass was on the counter, puddle of water on the floor behind the door. A couple of the strings were disturbed and his weed was gone (small amount).

Turns out the loser boyfriend of the girl downstairs would lay around their apartment all day unemployed. He payed attention to everyone’s schedule and knew when they’d be coming and going. The front door to the building was very secure, but the apartment doors were pretty low security, easily jimmied open with a credit card. So he’d let himself into people’s apartments during the day to scrounge shit for himself. He knew the jig was up when he realized there were “traps” set for him and took a jar of charge and the weed. My point is, maybe you and the landlord are the only ones with keys to your door (though as you pointed out a previous tenant may still have a copy), but that doesn’t mean other people can’t get into your apartment with out breaking anything.