I think someone was living in my apartment while I was at work

I know how this sounds, but I genuinely don't think I'm imagining it. A few weeks ago I started noticing small things being moved around my apartment. Nothing major. A kitchen chair would be slightly pulled out, a cabinet I'd left open would be closed, things like that. I blamed myself because I work long hours and sometimes come home exhausted. Then I noticed my bathroom mirror kept having a small handprint on it, even though I live alone and hadn't used the bathroom that morning.

The first thing that really bothered me was my bedroom window. I always lock it before leaving because I'm on the ground floor. One afternoon I came home and the window was unlocked and opened about an inch. I checked the whole apartment and nothing was missing. I changed the lock and thought that was the end of it. Two days later, I came home and found my bedroom door closed. I always leave it open when I go to work. I started getting paranoid and put my phone on an old tripod facing the hallway before leaving.

The next morning I checked the recording. For almost eight hours, nothing happened. Then at 3:17 AM, the camera picked up something that made my stomach drop. My bedroom door slowly opened from the inside, even though nobody was visible in the hallway. It opened about six inches, stopped, then slowly closed again. I thought maybe there was some weird pressure or airflow, so I checked the room. That's when I noticed the mattress on the side against the wall had a slight indentation in it, like someone had been sitting there recently.

I haven't slept there since. I told my landlord and he changed the lock on the apartment door, but he thinks someone probably had an old key. What bothers me is that nothing was ever stolen. Whoever was doing this wasn't looking for valuables. They were opening my window, moving my things, closing doors, and apparently spending time in my bedroom.

Last night I watched the camera again and noticed something I missed before. At 3:17 AM, just before the bedroom door moves, the camera microphone picks up three very quiet knocks. They aren't coming from the hallway.

They're coming from inside my closet.