Apple's "Find My" Network Officially Open to Third-Party Devices
What, wait what?
The data is all encrypted and anonymized to ensure there are no privacy issues.
The phone doesn't "connect" to the missing device over Bluetooth, it just "sees" that it's nearby. All Bluetooth devices broadcast their ID constantly while on; that's the only way phones can find them and connect to them. And phones regularly scan for nearby devices and "see" their unique IDs. That's the basis for how Bluetooth works. As long as you have Bluetooth turned on, your phone already knows the unique IDs of all Bluetooth devices nearby.
Apple's system simply takes advantage of that scanning that your phone is already doing as part of any basic implement...