cat-localizer VS room-assistant

This is the way I would do it. You can even track where in the house the item is.

They are ESP32 with builtin power supply, I use them for almost everything. Temperature sensors (with the addon!), heater control, cat locator and, yes, for power monitoring. I set `PowerOnState` to `4` in Tasmota. Relays usually fail because of switching cycles, being always on is much less taxing.

I agree that gutting a chip reading door and using the mechanism to hold down the curtain of your door might be a viable solution, but to answer the actual question: https://github.com/filipsPL/cat-localizer

Look into cat-localizer, it uses BLE beacons.

The first time I read the post I also assumed the phones are the beacons and was about to suggest the cat-localizer, but actually OP is asking for the phones doing the tracking.

finally, a way to locate my cat in my house

Also, I am using the bluetooth on the Pi to drive Room Assistant, to tell which room I'm in and automatically switch on the audio in that room when I am in that room, and switch it off when I leave. It also has support for the grid-eye sensor which looks like the flir sensor you useThere's also a similar project for ESP32 called ESPresence.

Check out Room Assistant. I have 9 RaspberryPi Zeros set up around my house and these do a great job tracking which rooms our cats are in, provided that we put a tiny BLE dongle on their collar.

Earlier I had six raspberry pi's spread around the house to try out new things (e.g. room level presence detection - which did not work well despite weeks of tuning - and as snapcast clients). But it became a pita to keep watch over so many independent systems, updating them, remembering the IP, passwords, forgetting what I have installed last time, etc. This is what I like with the whole docker thing: fire up a container, tweak it, and then if it is not used delete it again without affecting the other services.

There's a project called "room-assistant", however you'd need to have multiple Raspberry Pis running it in various locations...

https://www.room-assistant.io/

You can try room-assistant. It takes your Bluetooth devices and calculates distance, works quite nice in my environment for in room detection.

I've been playing around with room assistant. https://www.room-assistant.io/