Large, low-flying, slow moving, silent black rectangle with no lights

This has stuck with me for years.  I have always attributed the sighting to the prolific amount of DOD and NSA activities in the immediate area, but its still weird.
I was walking to my friend's house after dark, and was rounding the NW end of Wilde Lake, where there's a walking path.  While walking up a slight incline behind some houses, I had an odd feeling of being watched, and instinctively looked up.  
What I saw was a blank space in the sky. The night was crisp and clear, and many stars were visible.  There was a solid rectangle shape, black, with no lights, and completely silent, blocking everything behind it.  It was way too large to be an aircraft.  I stared at it for 30 seconds or so, it seemed to be almost imperceptibly drifting to the north.  I looked down to get my bearings, and when I looked back up, it was gone.
Witness indicates that the date is approximate.  PD