It is understandable, but an error of logic, to assume that an open window and shutter mean abduction.
Kate and Gerry would know.
They knew what Madeleine was capable of.
They knew if she could have opened the window and the shutter
They would know if she would have dared to jump out into that black area behind the wall.
They would see if the bed under the window was freshly made, or creased
They could see if there were any marks on the cill etc.
They would know if she would even dream of leaving CCat.
They would know if she would waken the twins first
Also, had she taken the back way out
She had never gone out thru the window before. so as intelligent parents, they would find it unlikely that she would chose to go that way. She had always used the patio door or the front door.
According to statements IIRC, the bedroom window had never been tried and the shutters not raised, since they arrived Please correct me if I am wrong.
They would know if she was capable of sliding the patio door.
They would know if she would close it again.
They would know if she would leave her comforter Ccact behind.
They would know if she was aware of where they were eating
They would know that she would have seen them the instant she left the actual indoor part to go onto the patio area. There was a direct view of the group from there, just 50 metres away in a lit outdoor restaurant
THey would know that she had a bog voice and was not shy of using it.
So much that they would have known and probably more. All stuff that we dont know.
Sure, they would have had a pretty good idea that she had been taken ... but they desperately wouldn't want to believe it