Re: distance and signal strength
Better would be to use sonar. Send an ultrasonic 'chirp' through the phone speaker and pick up the reply via the microphone. The round-trip time of ~12ms is rather more manageable, though having the microphone live all the time would not be easy to sell...
Better to use a one-way ultrasonic pulse. Phone sends an ultrasonic burp and a Bluetooth broadcast at the same time. Bluetooth signal arrives at listening phone more or less instantly, and distance is proportional to delay before corresponding acoustic signal arrives.
The microphone doesn't have to go active until Bluetooth broadcast received, and even then only has to be live for a hundredth of a second. If the burp takes more than a hundredth of a second to arrive, then the phone is more than 3 metres away and we don't care about it.
Biting the hand that feeds IT
