I saw a grey rectangular patch against blue sky, thru binoculars.

While watching an ultra light aircraft demonstration, using binoculars, I saw almost directly overhead a light grey rectangular shape against the blue sky. I was looking almost straight up. The rectangular shape had clearly defined edges. The long part of the rectangle was oriented east/west. I looked back at the shape several times in the next few minutes. I always located it quickly in the same spot. It looked like a patch on the sky. There were other spectators around, but none with binoculars. I didn't mention it to anyone, because the demonstration had everyone's attention. After a few minutes, I became very concerned that there was going to be a violent accident with the ultra light aircraft, because the pilot was trying aerobatics in the flimsy craft. I left in my vehicle. I have never mentioned this incident to anyone, nor have I ever again seen anything similar. I am a 69 year old retired banker, a former computer progammer and I was a control tower operator in the Air Force in the late 1940's.