I’d like to add some “real” presence detection sensors into some of my rooms. Not only motion sensors, but presence detectors, which also detect if someone isn’t moving (like sitting on a table working/reading, …).
presently available tech stacks are:
I’m open to others also, if they’re reasonable with some kinda hardware hubs or stuff.
Are there some good ones available on the market, which aren’t over-the-top expensive! 
Besides the usual motion sensors there are things like room-asssistant which could be run on a Pi Zero.
I might wait though for the new Xiaomi device announced as this seems to be finally (?) a real solution for in room presence detection. Not cheap though.
There are some radar based presence sensors for up to 200€/$, and around 60€/$ for the FP1 doesn’t seem expensive here - if it really meets the expectation.
…but the room-assistant thingy brings me to an idea: my usecase is to track human presence in specific rooms for heating regulation - and in 99,99% of all time, my family wears their smartphones like a part of their body… so, simple Bluetooth tracking from an Pi (I’ve got some lying around here) or even a ESP8266/EP32 would be another (bridging) solution. Thanks for the hint!
Compared to those yes, compared to the normal motion sensors its a lot.
But agreed, this is comparing apples and oranges kind of.
I am looking forward testing the FP1 once available.
I do have a room-assistant instance running as testing since about 1 year and it surprisingly works well.
From an ESP point of view, there is also this post here, maybe of interest as it uses the same functionality.
is Aqara FP1 supported by xiaomi binding?
Here is a interesting video about this new type of sensor.
Here is a review about a cheap Tuya device with this sensor:
https://blakadder.com/cheapest-tuya-human-presence-sensor/
Like Andreas says in the video, there might be cheap and good devices in the near future on the market.
I will stay tuned for this. 
Anyone tried one of these in DIY? I’m still looking, btw! 
This looks absolutly great:
But can we add it to openhab 3 as well?
as it’s running on ESP32 - ESPHome, you can configure, what’s on your mind. I’d bet it already has some MQTT-based communication already ootb.
Then use MQTT to integrate it.
Problem: seems all waves already sold out, but you could check for the components and put them together yourself.
Are you still looking for presence sensors? I just bought a couple of Sonoff SZNB-06P sensors that are promising.