Howdy,
From the Find My User Guide
• Find My network: Turning this option on helps you find your device (using Bluetooth) even when it isn’t connected to a Wi-Fi or cellular network.
Note: When you turn off the Find My network, your Mac can’t be found by you or anyone else.
And from the developer pages.
What is the Find My network?
The Find My network is a crowdsourced network of hundreds of millions of Apple devices that can help users locate a missing iPhone, iPad, Mac, Apple Watch and soon, third-party Find My network-enabled accessories, using the Find My app. Devices in the Find My network use Bluetooth wireless technology to detect missing devices or items nearby and report their approximate location back to the owner. The entire interaction is end-to-end encrypted, anonymous, and is extremely data and battery efficient.
It sounds like your (lost) device can send out an encrypted low energy Bluetooth beacon to other Apple devices nearby that then send their location to Apple Servers and thus an approximate location of your device. The more devices that send in the location, the more accurate the approximation is for your device. This is all encrypted so no one but the owner (you) can "Find My" (your) device. Since it uses Bluetooth you don't need WiFi or Cellular networks for it to work giving more chances to find your device.
You turn it on or off at
System Preferences > Apple ID > iCloud > Find My Mac > Options > Find My network [on or off]
ivan
