I have seen some setups using bluetooth for similar use cases, but it will probably involve some hacking. The phones you want to detect are typically not in discoverable mode.

If the phones use wifi, you can probably detect some proximity, but this will also probably mean you will have to scan for them at a rather low layer, since they will not access your wifi antenna, and they will probably connect encrypted. Have a look at kismet for some low level wireless bonanza.

The easiest way to do it, though, I would guess, is to use the camera module and a panaramic mirror.

I have seen some setups using bluetooth for similar use cases, but it will probably involve some hacking. The phones you want to detect are typically not in discoverable mode.

If the phones use wifi, you can probably detect some proximity, but this will also probably mean you will have to scan for them at a rather low layer, since they will not access your wifi antenna, and they will probably connect encrypted. Have a look at kismet for some low level wireless bonanza.

The easiest way to detect whether someone is in a room or not, though, I would guess, would be to use the camera module and a panaramic mirror.