My thought is that there must be some way to track AirTag through the signal it sends. It must somehow shake hands with devices registered in Apple’s “Find My” network as part of the handshake.
Track AirTag manually
If you are doing a Bluetooth scan of AirTags, one thing to remember is that your ID (a string of letters and numbers that looks like 12: c9: 34: f8: a1: …) will be changed to a different one, The value is random. That was intentional. Your goal is simply to find the location of nearby Bluetooth devices that are transmitting. You will not be able to specifically recognize the AirTag (unless it is not paired with an iPhone, this kind of randomization will be disabled).
I don’t have my AirTags yet, so I can’t discuss how to scan in more detail, including the possibility that ordinary people can successfully complete the scan. In other words, using a Bluetooth scanning app to search for unwanted AirTags shouldn’t be your preferred method of ensuring privacy. You can find a lot of red herring to begin with, and finding AirTag isn’t as easy as finding a small device that won’t stop tweeting.
On the contrary, if you are an iPhone user, you should definitely install iOS 14.5, if this problem occurs, it will notify you passively. If you are using Android and in doubt, turn to the scanner while waiting for the AirTag to beep (assuming the person controlling it is not around). And by crossing your fingers, Apple offers a more powerful solution for those who want to know that AirTag is tracking them, rather than a week later.