It is interesting.....hard to make out what it is so therefore is 'unidentified'.... but could easily be another plane.
Your right Zandore that the pilots would be highly trained and would have spotted it on radar if it was simply another plane...also would prob be told about it by air traffic control.....but how do you know they didnt see it and weren't told about it?.... theres no interview on that site from the pilots or air traffic reports from that day which would surely be the first thing to look look into to identify what it was.
Be interesting to find out if there was another plane in the vacinity that day...Can then rule out whether its a plane or not.