I feel that usually, the people who grab stuff is more of an opportunist who sees something and then performs the "dash and grab". Things like gas cans, 4 wheeler ramps, propane tanks takes, etc...
I personally have had a couple hundred dollars stolen from my tent trailer while me and a buddy were out hunting. We were camped in a campground and there were many other camps in the area. We had a pretty good idea who it was but could not prove anything. Just counted it as a loss and lesson learned.
I once had an acquaintance I met while hunting Utah. I'd see him and talk to him in the course of several years during deer season. He was an out of state hunter like I was. He would tent camp high on top of a mountain that I hunted on. One season years ago I ran into him as he was packing up what was left of his camping stuff. Seems some locals did not like him camping in their area and trashed his equipment and crapped on his cut up tent. I guess to send him a message.
Although this is rare and never happened again to my knowledge, it has always stuck in my mind and I have become very cautious. I really think these are isolated incidents and usually does not happen that often... As Rcfireninja stated above, knock on wood!