I get the concerns about leaving your site unattended. But, at RV parks (at least the ones I've been at), they assign you a site, so no real opportunity for someone to take your spot. State parks, most decent folks would honor the tag on the pole.
But, 4 thefts in 10 years, even if that doesn't sound like alot, I would have to come up with a different solution after the first one.
I'll not side totally with the folks who blame the victim, BUT, I also believe in taking care of ones own stuff. It's unfortunate, but that is the society we live in today. I too would like to know the culprit's reaction when you confronted him. Did you actually get to that point, or just left the note?
Mike