Apples Find My Feature Uses Some Very Clever Cryptography Wired
Andy Greenberg writing for WIRED
The solution to that paradox, it turns out, is a trick that requires you to own at least two Apple devices. Each one emits a constantly changing key that nearby Apple devices use to encrypt and upload your geolocation data, such that only the other Apple device you own possesses the key to decrypt those locations.
Stuff like this is why I like Apple.