Do we even know if the window was opened from the outside, or if it was opened from the inside. Nor do we have any idea why it was opened at all.
We don't even know if it was ever opened.

We do know, however, that the relatives and friends believed that the shutters been jemmied, forced, smashed or broken. Some of them thought the doors were locked. some of them thought the police were doing nothing. The myth of the police doing nothing is still being spread all these years later.
Trish Cameron;
The door was open and the window in the bedroom and shutters were jemmied open.
the front door was lying open; the window had been tampered with; the shutters had been jammied open... or whatever you call it, and Madeleine was missing.
Jon Corner;
"She just blurted out that Madeleine had been abducted. She told me, 'They have broken the shutter on the window and taken my little girl.'
"They had left the apartment locked while they were having their meal, but when they went back the last time they saw the damage."
"She's also very upset that
the police don't seem to be doing more to find Madeleine. She thinks there's too little happening."Jill Renwick;
"The shutters had been broken open and they've gone into the room and taken her."
"I spoke to them this morning and they said
the police had done nothing overnight and they felt as if they'd been left on their own. They just don't know where to turn."
Philomena McCann;
"Some people may ask why they left the children alone in the apartment
but it was locked and they had a full view of the front door and they were checking every half hour."
Brian Healy;
"Gerry told me when they went back the shutters to the room were broken, they were jemmied up and she was gone," said Mr Healy. "She'd been taken from the chalet.
The door was open."