How many parents go out for 2/3 hours during the hours of dusk/darkness and leave their children in an unlocked holiday apartment where a) they don't have direct line of sight of the unlocked doors and b) others do have direct line of sight of the unlocked doors?
Other members of TP9 did not leave their children in an unlocked holiday apartment:
Payne's apartment was locked and fitted with a hi-tec baby monitor
ROB/JT apartment locked - checks carried out via front door
Oldfields apartment locked - checks carried out via front door
McCanns apartment unlocked - checks carried out via patio door
Even if one considers teenage parents who might have come from chaotic backgrounds and experienced poor parenting themselves I doubt they would be so stupid as to leave such small children in an unlocked holiday apartment unsupervised. Moreover others unknown to the McCanns would know their small children were home alone in an unlocked holiday apartment. The McCanns were lucky not to find themselves locked up for child neglect. Reading KM's book I find her disagreeable. That said I do not believe they were directly responsible for MM's disappearance.
