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.
I’ve got an app that does this, it will detect them and send a notification but you can just ignore it
though I am very tempted by the little button that makes the device in question play a sound