It is quite possible if the had friends in the cabin next to them and they were adjoining, that they could have entered that cabin, locked theirs from the inside and then missed the ship in port. That is quite possible.
Except after my investigation I have determined there are no connecting cabins on either the upper deck or here on the Verandah deck. If they fell from the Verandah deck, they would fall to the deck below. SO, they had to be on the upper deck and it appears the balconies do not open up to each other either. SO they either jumped, were having hanky panky and accidently fell over or were left in port somewhere. I am sure they already know if the left the ship and got back on or not.