When I leave a 101 trailer connected to the TV out on a corner far from sight (IE with a "For Sale" sign), I run a chain from the trailer drawbar to the TV receiver with a padlock.
I also don't use extended safety chains normally, so you can't unhook the safety chains unless you turn the combination hard in one direction, or finagle it just right after unhitching (I have it down to a science now, but it's not quick or easy).
If the trailer's loaded and heavy, I don't really bother with a lock. You need more and bigger tools to unhitch it at that point if I don't have a crank landing leg on it. Or, you have to reposition the load, which still won't make maneuvering by hand easy.
When I have multiple trailers parked unhitched, I run a chain and padlock through the frames or leaf springs if I'll be away for any longer than usual workday periods. I also have camera's on the approaches to my house, which is coincidentally where some trailers are parked. Hey, I'm from NY, people break in to your house to take food out of your fridge...
As far as trusting other people to notice shady activity, I've sold a few of these trailers in parking lots. No one has ever said anything if they even noticed a couple guys unhooking a trailer from a parked truck and moving it to another truck... Most people don't expect to witness anything worth reporting or interfering with, and use of tools almost legitimizes shady activity to strangers. How many times have you seen someone working on a car and thought they were trying to steal it?
edit: Forgot to mention, another way I've locked the lunette when unhitched is to wrap the safety chains through it and padlock them. That requires at least a cutting tool. That's usually how I park them when I visit my dad in NY...