I saw the Google IO video about the new Nearby API and thought it might be a great addition to HABdroid.
I am not 100% sure if I understand this correctly, but especially the ultrasound feature seems great for presence detection, because it does not go thru walls.
I imagine configuring a raspberry pi to send a special unique ultrasound every few seconds.
HABdroid could receive this signal and sends “I am home” to the openHAB server.
The same might be possible via Bluetooth LE and beacons.
See also: Integrate Googles new Awareness API