My question is: suppose the drapes hadn't been open. Was the light visible through them from outside? That may have been a better clue to the gunmen, assuming they (or an accomplice) was keeping track of MC's whereabouts, and the hit was planned for when he would return to bed. They sprayed the whole room, not just a sniper shot at Michael. So, ironically, if the drapes had been closed, Michael may have undressed and lingered around just long enough to be shot to pieces. But that last word from Kay triggered his instincts in the nick of time...As to who did open the curtains: put yourself in Anthony's shoes. He's waiting for his Dad to come up and see the drawing. He may have pulled the cord to get a look outside to see where his Dad was, or maybe to take a peak at some sounds he heard, whatever. A little boy's curiosity could be the unknown variable that the plotters could not plan on. (I wish I knew the outside of the house better)
I think the whole scene is author chosen for effect, not realism.