My trick for overnight crabbing is to drop my pots around moored boats. Near where I live there is a good crabbing area where boats are moored almost all year long, with people coming, going and overnighting on their boat. My thinking is that, unless the thief sees you drop the pot and leave (certainly possible), he will think that it's possible that the trap belongs to someone in one of the nearby boats and is watching the pot and so won't mess with it. At least that's what I tell myself. I also use cheap, collapsable traps so that if they do get stolen, it's less of a hit.
As an aside and with regards to traps being picked or poached, but not stolen... From time to time I pull up a pot that is cleaned out. No, crab, no bait left except for a few bones, nothing. And this in usually productive grounds. Did someone poach my crabs? So, now I shut my pot in a very, very particular way, so I KNOW if someone else opened it. The result? Whenever I've pulled up an empty pot, there has never been an any evidence that anybody tampered with the pot in any way. Not once. Interesting. Except for the time when my friends played a joke on me, took my crabs and left a bottle of tequila in the trap!