https://www.wired.com/story/apple-find-my-cryptography-bluetooth/
In upcoming versions of iOS and macOS, the new Find My feature will broadcast Bluetooth signals from Apple devices even when they’re offline, allowing nearby Apple devices to relay their location to the cloud. That should help you locate your stolen laptop even when it’s sleeping in a thief’s bag…
“Now what’s amazing is that this whole interaction is end-to-end encrypted and anonymous,” Federighi said at the WWDC keynote. “It uses just tiny bits of data that piggyback on existing network traffic so there’s no need to worry about your battery life, your data usage, or your privacy.”..