What explanation? Madeleine was there and all was well. It's a hindsight observation.
No Eleanor, that doesn't work
Gerry goes into the apartment and sees, immediately, that the children's bedroom door is open wider than he left it half an hour before ( and he knows that no-one else has been there in the meanwhile )
So concerned is Gerry about the open bedroom door that he does not even look into the room ... he immediately assumes that Madeleien must have woken up and gone into his and Kate's bedroom
And that's where he looks first
Having found that the explanation for the open bedroom door was
NOT that Madeleine had woken and gone into their room Gerry does what anyone might have been expected to do in the first place, and
looks in the children's room
There was Madeleiene, safe and sound ... and beautiful ... he says
So what happened then ?
He
forgot about that open bedroom door ?
It didn't matter anymore ?
The fact that there was now no explanation for it ( Madeleine having not gotten out of bed ) did not alarm him ?
He just had a pee and went back to the tapas bar without another thought ?