Gerry and Kate were on holiday, Madeleine said a sweeping remark that they didn't come when she cried then went off to play. Madeleine was sound asleep when Gerry did his check, so not awake and crying. He found the door open more than they had left it, so thought maybe Madeleine had got up and gone back to bed. Why would that be foremost on his mind when he went back to the Tapas bar? Madeleine was asleep in her bed, he wouldn't have been thinking 'an abductor had come into the bedroom' with hindsight it's easy to think he should have said something.
So you go out every evening leaving your children home alone. You decide that looking in on them every half hour is satisfactory. Then your daughter makes a comment which suggests that something happened during one of those half hours; your children cried and you missed it. Even worse, your child noticed you were absent.
So what is your response? According to your friends you left a door unlocked so your child could find you if there was more crying; even though you didn't really believe her;
"Could Madeleine and Sean have woken up while we were at dinner? If so, it was worrying, obviously, but it didn’t seem very probable." [madeleine]
According to their spokesman Clarence Mitchell;
He said Kate and Gerry discussed the matter and and decided it was safe to go out to the tapas bar on the evening of May 3 once the children were asleep - but they would be more "active and diligent" in their checks.
https://www.mirror.co.uk/news/uk-news/kate-and-gerry-mccanns-fury-over-302522What did that mean? The same half hourly checks they had been making for the previous 5 evenings, apparently. During the first of those checks a door appeared to have been interfered with. Was it the child who moved it? Apparently not, she was sleeping soundly in exactly the same position she had been in 30 minutes earlier. Gerry therefore went off without solving the mystery, and without giving it another thought, apparently.