I certainly understand being upset because I don’t take other people’s things and claim them as my own, so I expect others to do the same.
I am often the last people, or close to it, in the barn at night. So anything that is left out and I know where it goes, I put it away. Saddles, helmets, etc. If I am unsure I just leave it. I have left some of my own items out and they’ve always been there when I’ve come back. Our barn has a ton of lesson kids, but most of them have help tacking up, so they’re not wandering around picking things up on their own.
If I was in your situation at my barn, I would first make sure I was not upset when speaking to the trainer/barn manager. Then I would let her know that some of my things have continued to go missing and I believe Barn worker took them. I would ask her to speak wit the barn worker about not taking things that don’t belong to her. If it continued to be a problem, I would expect that person to no longer work at the barn.
Is this person taking other people’s things, or are your the only ones you know of going missing? I would want to know that as well before going to the BM.
I feel like realistically, as long as there are boarding barns, there will always be someone with sticky fingers and people who aren’t good about putting their stuff away. Maybe not at all barns, but I believe those people will always be out there.