Using bluetooth as a proximity sensor.

Ok good point. But I think you will find that your phone doesn't advertise itself continuously like the Puck does. My android phone, for example, only advertises when the bluetooth settings page is open. So the iPhone maybe isn't hiding, per se, but is just not advertising. Another possible issue as I also have with my phone, is it doesn't advertise as a BLE device. EspruinoHub for example doesn't see it. But from my PC I was able to add the phone as a Bluetooth device and send files and play music from it. Maybe you could do it in reverse and detect the Puck proximity from the phone, and then take some action.