Last November, I cruised on board the Ruby Princess. Every time you get off and on the boat you are checked out by the princess medallion. It’s an electronic disk worn on a lanyard, around your neck.
You can’t do anything on board without it. It opens your room, never needing a key or a card. It keeps your account information for drinks and food. It checks you onto the ship and off. We often joked about detecting anyone going overboard.
Could he have just gotten off at a port of call and not got back on board?