I wouldn't dare correct you, Sadie.
Good, but I didn't think you were a wimp?
But you know what the problem is. Not only did nobody see the window open before 10pm, but they also seem reluctant to say they saw it open at 10 (or shortly after).
How could they not see it?

Several reasons are possible lyall.
It was really dark in there.
Crossing from the car park entrance to the end of the walled pathway every flat tenant had to use, meant that the eye of the walker going in or out was always at a point that the periferal vision did not pick up the window at all. That is assuming that the walker was looking at the light in the direction they were walking
Once in the walled pathway it was curved so that even when walking towards that window, the window was out of sight behind the curve of the apartments walls.
No-one walked past the lobby to the other apartments .... so no-one ever noticed whether the shutters were raised or not.
Additionally the shutters are very close in colour to the walls of the building, so in poor light would likely blend with each other
Please also notice that the bottom of the shutter is hidden by the wall. Do we know how far open the shutter was? Maybe it was only open a few centimetres and the opening was out of sight anyway, behind the pathway wall?