In a collaborative effort, Apple and Google have developed an industry-standard detection feature called “Detecting Unwanted Location Trackers” (DULT) for Bluetooth trackers. This standard allows users on iOS and Android devices to be alerted if an unknown Bluetooth tracker is monitoring their location.
The service already excludes any geographical tracker that is within range of its owner (as determined by whether the owner’s primary device is moving with the tracker).
They could probably use a few other rules, too, like excluding trackers that are moving with more than 10 other people simultaneously, so that some keys left behind on a bus, train, or plane don’t trigger the alert for a bunch of strangers.