It's an interesting detail. Hypothetically if a criminal did open the shutter and window from outside, and then pulled a curtain aside, do you think he would be able to see, from outside, anything inside the room?
S/he would be listening and looking into a backlit room with a shard of light entering via the ajar door. Madeleine's bed would be in shadow and it would not be immediately apparent that it was occupied. One cot had an opaque back therefore the child sleeping in it could not be seen ... dependent on the angle of the light entering it is possible a soundly sleeping child might have been seen.
I don't think a person observing the room from the window and holding the curtain back to do so would have had a very clear view of the interior.
Had the window been opened from the outside with the intention of entering the residence I am of the opinion entry would have been effected.
I don't think that is what happened. I think a key was used.
However, had an intruder entered via the patio door and been disturbed when Madeleine's father unexpectedly turned up ... it might have been opened as an escape route ... but I don't think it was an escape route either.
If the patio door was the entry ... the wooden door was the exit ... the open window being the contingency emergency exit.