Unmarked Boeing 767 Approaching Dulles International Airport

I live close to Dulles International Airport. Today I was walking on a trail directly on the flight path of aircraft leaving and approaching the airport. I saw a Boeing 767 approaching that was conspicuous because of its total anonymity. It was overall white, with just a thin cheat line over the passenger windows. Though it was very low, I could observe no other markings at all- no registration numbers, no call signs, nothing. It definitely did not look like a military, commercial, or civilian aircraft. I called the operations center/control tower to see if I could find out anything about the aircraft, and the person to whom I spoke merely said, “Some of the aircraft that use this facility are unmarked for good reasons.” That’s all he told me, understandably. Would anybody care to speculate as to its owner? There are obvious answers, such as the CIA, NSA, or other “spook” outfits, but it’s curious to me that any of these would fly a plane secret enough to be unmarked into a public airport such as Dulles.