Let's build a Bluetooth proximity detector! You only need a Raspberry Pi, USB Bluetooth, and a phone with Bluetooth enabled. This tutorial will show how to detect a known Bluetooth device that is within the range of the Raspberry Pi without pairing the devices.

Step 1. Ensure USB Bluetooth is working on the Raspberry Pi.
Install sudo apt-get install bluetooth
Check it Bluetooth status with this command: hcitool dev
Should return the device's Bluetooth address: Devices: hci0 00:00:00:00:00:17

Step 2. Getting the MAC address of a device.
On an Android Phone version 6.0.1, Settings > System > About device > Status > Bluetooth address.

Step 3. Test Connection to Client Device
Command: sudo l2ping -c 2 00:11:22:AA:BB:CC
Result:
Ping: 00:11:22:AA:BB:CC from 00:00:00:00:00:17 (data size 44) ... 0 bytes from 00:11:22:AA:BB:CC id 0 time 7.32ms 0 bytes from 00:11:22:AA:BB:CC id 1 time 22.46ms