Imagine this scenario:
The parents are in the hotel room with their kids, when a freak accident occurs, killing Madeleine instantly in the process. Like she falls off the bed and breaks her neck, or even one of the parents smack her and she lands awkwardly and dies from it. Now the parents instantly realise how bad it looks and that they could get the blame for her death. And ultimately lose their other kids in the process. So they hide the body by moving it outside the hotel somewhere. That night they have plans with friends to go for a meal, not wanting to look out of sorts they decide to go ahead with the plans resulting in setting up circumstances that could indicate kidnapping as the cause of her disappearance.
So knowing that the body is not in the hotel, they go and have their meal. When they return, on no she's missing, someone has kidnapped her. Police and media get involved, and it snowballs into something a lot bigger than planned.
Now this is one of the scenarios I've had in my head for a number of weeks now and when I heard about the possible blood on the wall, I thought something similar to what I thought must have happened. Or maybe I've got an over active imagination.