In previous threads I've read on this topic, some surmised that a portion of the theft wasn't intentional theft, but rather people mistaking the goodies hanging outside their cabin for free treats given away by the cabin's occupants. I imagine this sign would be more targeted at that group and less at the people with no morals or decency. Plus, a huge percent of the passengers on a cruise don't even know what FE is and there is a lot of curiosity about it. The first time we cruised I guessed that maybe people just hung their FE hanger outside their door like hanging stockings the night before Christmas and strangers or CMs on the cruise dropped treats in it. I had no idea what it was and didn't know much about Disboards or the other FB groups so I had no idea anything could be organized with other cruise passengers like that. Maybe reading that it's an "organized gift exchange" could deter some people from taking things?
Zippers are a good idea but most of the FE gifts we gave and received stuck out of the pockets and zippers would not have been able to be zipped.