It's interesting to me to discuss the tracking part of the medallion when we pretty much all walk around with a tracking device in our pockets every day. If that is the concern, as someone mentioned earlier, stick it in a cheap RFID blocker. And as someone else also mentioned, there are CCTV cameras that cover every public area on the ship and can see where you have been quite easily.
I also wonder if they have geo-fencing, especially at sea, that would notify crew of a medallion (and possibly its owner) went overboard.