For those that are hanging on to the fact of him wearing the same clothes day after day, he could have managed to get into the crew area where laundry is processed and obtained a change of clothes or taken some from a laundry bag left in the hallways. He could even still be on the ship, again if he managed to attain access to the crew areas there are a lot of areas which can be hazardous just as trying to hide under a dumb waiter (food lift) and ultimately getting crushed in there.
If he managed to obtain a crew uniform he could easily trail a legitimate employee through a secured door.
Another angle is he is with a crew member, there are many crew with solo cabins who could be harboring him in their room.
I have no evidence to any of these, but just some ideas as to how and why he may still be aboard the ship.
For those looking for a sinister plot, maybe he pissed off a cook, telling them how the food on CCL sucks, and so the cook chopped him up and fed him through the waste food disposal system.
I'm sure no DNA testing was performed in the kitchens in the short few hours the ship was in port.
Ok, that is more than enough for the novelists in here to write a book about!