Read up on the Apple AirTag and see if that will work for you.
I think if someone with an apple phone steals the trailer, the Airtags will alert them a device not registered to them is close. They may not find it, but it may alert them.
I doubt that is the case or you would be getting pings all the time from ones that don't belong to you. A lady I work with has one in her purse and my phone has never alerted me of it.
Maybe because it's near her and not moving with you? Ask her to put it in your pocket and walk outside away from her purse and see what it does. Not sure how long or how far away from the owner it has to be before it alerts you of being tracked.
https://support.apple.com/en-us/HT212227 That is interesting, if she has one with her today I will see if I can take it on some errands I have to run and see what it does.
In the event that you are using one of these for a trailer or car things should happen pretty quick, likely before the thief knows an air tag is with it, or can find it if they do. Some friends of mine and I put air tags on our kids while we were at a crazy busy place. At one point we left the hotel and my son did not have his air tag with him, we had barely pulled away from our parking spot before it told me that I had left an item behind, so roughly 2-3 minutes from leaving the room to getting a notification that it wasn't with me. So if you set a trailers location you should receive an alert rather quickly that is has moved from its spot(unless you are with the item). I know if I am at work and I get an alert my trailer or car is moving I am going to make a phone call to the police pretty damn fast.
