I'm glad I saw this thread. I have the same problem concerning a certainly very small prop plane that takes off at about 1:30 AM from DCA just about every "morning". The unfortunate thing is that this plane is so slow it takes about 5 minutes to be out of hearing. I have watched its lights a few times and wonder "how can that plane go so slow". I thought it might be the plane of an airport employee. I guess I will never find out.