Can you give a plausible, logical explanation for why a man sent to check on someone else's child would, on finding her missing from her bed, go looking for her and then on failing to find her, not mention it to the child's parents on his return to the table? Or - why (if he did tell the parents and therefore everyone else at the table) they all decided to lie about this?
You are assuming that speaking to one of Madeleine's parents would involve everyone at the table hearing the conversation. There were individual conversations taking place however;
4078 ”Can you remember what conversation was going on in the time before Kate discovered Madeleine missing?”
Reply ”Well again now, because err you know
when you’ve got a group of nine people you’re not all talking…”
4078 ”No.”
Reply
”In the middle of the table, you know, I would be, I was probably talking to Russell when he came back you know and talking about the fact he’d got a new steak, he didn’t get a warmed up one.
https://www.mccannpjfiles.co.uk/PJ/DIANE-WEBSTER-2.htmSo Matthew could have asked one of the parents where Madeleine slept without being heard by everyone. The answer would have reassured him if he hadn't checked that bed. If he had, then that parent may have decided to check for themselves; especially her mother, who had told her friends earlier that the patio doors were left open so Madeleine could get out.
So the disappearance could have been discovered at 9.30pm - 9.40pm, even if the alarm wasn't raised until 10pm.
This hypothesis is IMO.