I am sure it has happened to me many times, though my traps look like this:
So the bait stays put. I pretty much assume that there are going to be people out there who will steal from others' traps. Unfortunately here in Texas so many people leave their traps so long unattended, and people know that, that it encourages poaching. Frankly, with unattended traps poachers can provide a service, because a lot of times crabs and even game fish languish and die in them because they are unattended so long. I volunteer every February with TPWD, during a 10 day closure period where all traps must be out of the water, and any traps found can be kept. I don't usually keep the ones I find, as I have enough and mine are in much better shape as I only leave mine in the water for 1 to 2 nights at a time. These abandoned traps tend to be corroding and covered with barnacles and oyster shells.