I use the yellow hitch locks, nothing is 100%, like they say, if someone wants it bad enough, they will find a way to get it.
There was an incident many years ago at a local paint store, they cut the hitch off the trailer while still connected to the truck and took the whole trailer, at noon (lunchtime) in broad daylight in a busy parking lot. I think they had a trailer or tow truck to move it out of there.
I am looking into things like the lowjack or other brands out there. Some are inexpensive but have a monthly fee which I don't need, I just want something that can track the truck or trailer if it is stolen.
If your trailer is there without at least a lock on it, there is a big chance that it will be stolen sometime soon, at least here in Texas, California, Florida and a few other states where the crime is high.