From my experiences, when a trap is pulled up empty and the bait is gone, it’s often an indication of many smaller crabs feeding on the bait and getting out through the sides of the trap. A crab that is around 5” or less in diameter can fit through almost anywhere on the trap - I’ve seen them easily crawl out from traps on deck. I have a crab trap with a small mesh (size of a prawn trap) that I experimented with along with my regular crab traps. When retrieved several hours later, the small mesh trap is busting at the seams with crabs of every size ... consistently 30-40 crabs in the trap of which very few are keepers ... safely released of course. And the bait is completely gone. The other regular traps typically have <10 crabs in them but they are all larger (usually >5.5” in diameter). Over the last year or so, I now zap strap my traps closed as a way of verifying whether they have been “pilfered”. The assumption is that anyone who steals crabs from the trap is likely not going to put another zap strap on when they throw it back in the water - so at least it will verify someone took the crabs. So far, my zap straps have been untouched. I have pulled up my traps on several occasions now to find the bait gone but the zap straps still securely fastened. So I know now, in these cases, the trap was inundated with undersize crabs that consumed the bait.