Bluetooth is everywhere these days. It's a radio communication protocol designed to connect us with earbuds, computer peripherals, other phones, etc. By design, the signal is very low power and this limits the useable data range to a few dozen meters. The good news for us is that phones can pick up the frequency at considerably greater distances and can trigger an alarm if a new signal is detected and that's how Proxus works.
We don't need to communicate with the oncoming device - we just need to know it's there so we can take quick precautions. No communication necessary!