In this case a headcount before leaving could kinda have been feasible as the ship only had 120 passengers - often it isn’t simply because modern cruise ships tend to be massive and have way too many passengers to keep track of - but from what I could understand they had an electronic system in place and for some reason she either wasn’t marked as having left, or had somehow been mistakingly marked as returned.

She was noticed as missing when they did do a “headcount”, during the evening dinner.