Oval object seen circling a B-727 approaching landing at Boeing Field, Seattle Wa.

I was looking out my front window over Lake Washington and Mercer Island on a Sunday afternoon, and saw a Boeing 727 heading south to land at Boeing field.  Not a surprise, until I noticed an oval shaped, dark object overtake the 727, and start circling the rear section of the craft.  It trailed and then circled it, forward of the tail section twice then went at high speed to the west and disappeared.  The plane took no evasive action.  The next day when I went to my job at Boeing(designer/drafter)  I promptly asked one of the engineers what the funny new aircraft was that Boeing was testing, and told what I had seen.  The persons I talked to quickly told me to hush up, and not say a word to anyone else about it; that Boeing did NOT LIKE for their people to see such things and I could be fired.  Interesting.  I saw a similar object about a month later, same drill, and said nothing.  This is the first time I have ever reported this.
Witness indicates that date of sighting is approximate.  PD