I think you a worrying too much as I've never had a set of ladders nicked leaving them locked up on the roof of a van yet.
Mind you after saying that I've had a few near misses when driving away after being side tracked and forgetting to put the ladder clamps on. Oops!
To be honest standard trade double/triple type ladders are so cheap now so if a set ever got stolen they could be quickly and easily replaced. Would be annoying for sure but still would be more an inconvenience than great loss money wise.
Life is way too short to be worrying about things like that as if someone is determined enough will have anything away including a van full of kit.