Because I'm now leaning towards a flatbed vs. a standard pickup (to open up my options for UTV size ranges), the empty flatbed with ramps exposed would just be sitting there for anyone to take. In the bed of a pickup, they'd be just as easy to steal, but would at least be somewhat concealed from anyone who wasn't actively looking for an opportunity. Would folding ramps fit behind the seats in the cab? I don't want an extended cab truck, but that might be a good reason to change my mind.
Well, if somebody really wants to steal your stuff, they gonna do it. With a flatbed, you should be able to get some tiedown/lockdown points somewhere, I'd say either go with a cable or chain and lock, or else measure out how thick the ramps are and go with a big-ass padlock. I know there are some with a 5" or more shackle length.
Look into ways that contractors lock down the 50 ladders they carry on the ladder-racks on their trucks.
Personally, I'd recommend Paclock brand locks. (No affiliation, just a fan). Pacific Lock Co, made in USA. Re-keyable cores, pick-resistant etc etc. I have mine setup so I can use 1 master key on my truck toolbox, hitch lock, trailer tongue lock, trailer toolbox lock, chain padlock, and I have a lock ready for whenever I finally get a nice toolbox for my P500.
Makes things a ton easier VS keychain fulla different keys, but just hope you dont lose your 1 key to open them all.