Everyone is focused on who opened the drapes, but maybe that is not the question we should be asking. Maybe it is who didn't close them. Assuming the Corleones didn't live like Elvis, its likely the drapes would be opened during the day and then closed at night. I'm not sure whose job it was to do this but if the Corleones had domestic help that may have been someone's job, which raises the question How deep into the family did Fredo's betrayal go?