At WWDC 2019, Apple announced Offline Finding, codenamed Blue Avengers, as one of the new features of iOS 13 and macOS Catalina. It allows users to find their devices even when their lost devices are not connected to the internet.
A background process called Search Party intermittently broadcasts and receives Bluetooth beacon signals so that every nearby online device running iOS 13 or macOS Catalina can relay the location of lost offline devices to their owners. Offline Finding even works if the device is asleep, in which case the device enters a "dark wake" state to broadcast a Bluetooth beacon signal. Apple uses public key encryption and rotates the public key to keep everyone's location information secure and private.
Beyond the ability to find devices themselves, Apple is rumored to be planning to compete with Tile and announce a small Bluetooth beacon device that uses the same Offline Finding technology as iOS 13 and Catalina devices. This small beacon device could be attached to personal items such as keys, purses or wallets so that the owner could find them even when out of range of the items. An ARKit "star" image discovered in the Find My app bundle hints at the possibility of using augmented reality to find lost devices or items, similar to Pixie Tracker.
Also according to code strings found in iOS 13, Apple may apparently allow these beacon devices to be "leashed" to an iOS or watchOS device so that the user would be notified when a beacon device is out of range of an iOS or watchOS device. Tile offers similar functionality, which it calls smart alerts and is only included with Tile's premium monthly service.
After upgrading to iOS 13 when it is released this Fall the Offline Finding feature is enabled by default if you have Find My iPhone and Bluetooth enabled. You can check that Offline Finding is enabled by going to Settings > Apple ID, iCloud, iTunes & App Store > Find My > Find My iPhone > Offline Finding.
