Depends on your threat profile and budget. Specifically if you're worried about your door opening, you can use a magnet and Hall effect sensor(or reed switch). Another thing you could do is PIR, but the cool kids are playing with 60GHz pulsed radar, which does presence pretty well (I've recently tested that the XM125 radar I just got can pick up my breathing and detects presence from the other side of 2x pieces of 3/4in sheetrock).
I'm definitely not a cool kid:) I was considering putting a camera in the flat and hooking it up to a visual motion detector but that would be a bit expensive probably (would probably need another server in addition to home assistant...)
Yeah that's overkill. But PIR and esp32s would work for the most part. (don't rule out the radar, it's only $50 bucks at Sparkfun lol I just got it working the other day, it's pretty awesome)
