(NWR) I need some advice about my cat. Someone keeps taking his collar. >:(

OK, so I am really mad right now. My cat came home and for the 5th time now his collar is missing. This has been very frustrating to me because that collar has all his information and his rabbis shot information. The first collar he had, we thought he just took it off because it was an easy release collar, we got that same collar back and yet again it was stolen. We got another collar and actually tied one of those thin metal ties to close it together. Then we got a regular collar and 4th time, it was taken! Then we brought, yet another tag and melted the metal to his flee collar and the collar is missing again! Someone keeps taking his collars off of him and it's making us really upset at this point! Anyone have any idea of what we can do to keep his collar on? Or a way we can catch the people doing it? Has anyone experienced this before? The unfortunate this is since our cat was a stray before we got him, he likes being outside. If he's inside for too long, he goes nuts.