So You Don't Lose Your Mac
The new Find My app on iPhone makes it easy to find your friends and your devices, and with macOS Catalina it's easier to find your Mac.
Normally finding a lost Mac is an issue since it won't be connected to WiFi and there is no LTE, but Apple now lets your Mac send Bluetooth signals to other Apple products that are nearby -- even when it is offline or sleeping.
From there, those devices send the info to Apple and to iCloud so that you can find your missing Mac. Apple says this is anonymous and encrypted, so not even they know the devices sharing the info. They say, "because the reporting happens silently using tiny bits of data that piggyback on existing network traffic, there’s no need to worry about your battery life, your data usage, or your privacy."
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