Using bluetooth as a proximity sensor.

RSSI is an indicator for that - look at proximity-and-rssi

Puck can do a find devices

NRF.findDevices(function(devices) {console.log(devices);},1000);

[
  BluetoothDevice {
    "id": "f4:38:88:d0:bc:5c random",
    "rssi": -55,
    "services": [  ],
    "data": new ArrayBuffer([2, 1, 5, 13, 9, 80, 117, 99, 107, 46, 106, 115, 32, 98, 99, 53, 99]),
    "name": "Puck.js bc5c"
   }
 ]

there are some nice cell-phone apps like "BLE Scanner" for Android

It defines three zones

• immediate rssi > -50,

• near, rssi between -50 and -75

• far, rssi between -75 and -90