Tasers, bodycams and holsters among other things made by Taser are constantly spamming advertising packets over BLE in order to be able to link to each other quickly (ie if a taser or gun is drawn, turn on the body cam). These devices are almost all made by one company and per the Bluetooth specification, the manufacturer ID is the first three bytes of the device ID, 00:25:DF: the address for Axon International Torture Devices.

Credit to Nullagent/rfparty for discovering the vulnerability. https://www.engadget.com/how-hackers-are-using-bluetooth-to-track-police-activity-140012717.html

A couple notes in practice: without dedicated hardware BLE advertising packets are very random, your phone can only sense one of the three advertising channels at once and it rotates though them. Additionally local band conditions (how many innocuous BLE devices are also around, WiFi, unrelated ISM stuff, other factors) can change sensitivity. BLE is a chaotic spammy protocol that likes to shout over itself. As a result, detections can happen the instant someone gets within ~100m of you, or they may not be detected for several seconds. Some departments may not even use Axon hardware at all, though the majority seem to. You may be able to use the WiGLE database to get a sense for your locale, but I haven't dug into that yet.

Further research: This has a lot of potential for protest/protected first amendment activities. As stated, the ability to cue an audio alert. Apps that can trigger audio/video recording when cops are very close (high RSSI). Apps that can alert a friend that lots of cops are very close and the user is unable to halt a countdown. Apps that can wipe/lock/turn off a phone if lots of cops are very close or around for an extended period of time (in custody).