Before the AirTags, we have trackers from Tile and Chipolo, among others. I had them both. And yes, they can be abused to track someone and there is no way you would know that you are being tracked - that was then and still the same now. Chipolo, I have heard, will join the AirTag network - so they can be revealed when used to track someone without permission, I thnk. I don’t know about Tile and the others.
If you find yourself a victim of unauthorized tracking using an AirTag, reporting it to both police and Apple can reveal who owned it - it is paired with an AppleID to work - (also turn it off by removing the battery, don’t throw it away yet). At least there is that information. On other bluetooth tracking devices, good luck finding the trackers and whoever is tracking you.