Locking the release arm might be a good thing, although I don't do it. When we pull into a truck stop or rest area, one of us is always either in the truck or very close around it so it's always in sight. I will be sure to add it to my walk-around though. Good idea to check it and the breakaway switch as well.
While parked at a campground, we keep our roll-up tonneu cover closed and the tailgate locked. The only way to get into the back would be to cut open the cover, climb into the truck, and wrestle that 200# pound hitch out of there. No quick feat, unless you are a tall weight lifter! And a cardinal rule of thieves is to get in quick, grab what you came for, and get out quick, with a little hassel as possible. Pretty tough under those conditions!