I think I have been pretty lucky and never had anything stolen. I have seen many other peoples things go missing though. Every thing to stealling fire wood out of the back of my freinds pickup in the middle of the night in a state park to beer missing from a cooler to two quads on a trailer in a gated secure camp ground (someone cut a hole in the fence and dragged the quads and trailer out over 500 yards in the middle of the night).
I keep my site nice and clean and always put most things away at night locked up. I also have an early warning Woodstock(rescue breed dog that can hear a treat bag being opened at a quarter mile) and a Coug(german sheppard-pitbull cross, intimidating loooking mussle head with a bark that sounds like a 150lb dog but actualy very gental) that let me know if anything is going on outside the trailer at night.
Years ago I did have a camping chair that grew legs. DW put in the camp site next to us to hold a spot for a friend. relaxing in our site waiting for our fellow campers to arive we see our chair going down the road. A little girl had "found it" and was taking it back to her campsite.
On theif deterent I seen years ago was a manquin of a man dressed up in fishing gear holding a flash light and looking into the back of the owners pickup. At 20 feet in the dark there was now way to know it wasn't just a prop.
My whole life camping I have only seen a few example of stealing. I am more woried and have had more stuff stolen from home then while camping. Most campers are pretty good people and look out for each other, its only a very few that would be malicus and like someone else said its often someone not even staying in the campground thats likely to be the cause of a theift.