The police are confident that no one could get in unless they have a masterkey or something given how sturdy the door is.

I literally just broke into a residential apartment building's basement tonight. Picked a nasty deadbolt loaded with several kinds of security pins. Those cops apparently have no idea how easy it is to defeat a lock.

Anyway, I'd say there are two possibilities here.

First, possible CO poisoning. Check your carbon monoxide sensors if you have anything that could be generating CO (like a gas stove, or a connected garage, or a gas water heater, etc.). CO has a funny way of making people do weird shit and not remember it.. it also has a funny way of killing people.

Second possibility is somebody is breaking into your home. Get a better security camera. If yours is pointed at the door that keeps opening and it didn't record the actual opening of the door, then it is worthless.

Whoever it is, they're amateurs. Probably a teenager but not necessarily. This seems likely though by the fact that they keep revisiting the scene of their crime and they didn't steal any valuables but instead went looking through a woman's underwear drawer.

I am almost certain your apartment is being broken into though.