If you have multiple fixed BLE receivers, you can do full trilateration. This project integrates into Home Assistant using HA proxy BLE receivers to track phones moving around in your house: https://github.com/agittins/bermuda
Weirdly Apple seems to have something like this built-in for finding an apple TV remote - but the same flow doesn't appear to be launchable for any other device
Maybe I'm misremembering, but for example my esphome proxies in HA will show nothing for ages at night while my phone isn't being interacted with.
If the phone is lost, it's probably not super awake lol
That's an entirely different technology, and if you look at it from an RF lens, a highly impressive one.
Make your phone alert when one walks into a room within several meters