Not surprising - they could use virtually any sort of tracker to do this - just put a magnetic key store underneath the vehicle with the tracker inside.
Was surprised about how easily they can reprogram the key though - you would have thought the carmakers would have put an encryption key requirement on the data port.
And not just cars. They can detect bluetooth devices just by walking by with an RF/Bluetooth detector, $35 on Amazon. Hiding your devices is useless with that...