Someone in the Mirror Forums put out this same theory. Mathew Oldfield, one of the Tapas friends, was the last person to check on the children before Kate went back to the apartment at 10:00. Unfortunately, all he did was listen outside- he didn't go into the room; therefore, he didn't notice that Madeleine was gone.
Someone at the table in the bar at one point said, "This isn't the way it was supposed to happen", or something similar. I'm guessing he/she may have been talking about how the disappearance should've been discovered.
This also accounts for the fact that Kate McCann made so many "mistakes" when she found Madeleine gone. Saying, "They've taken her", is not a normal way to phrase the discovery that your child is missing and leaving the twins alone when an alleged kidnapper is around makes no sense either.