Flat oval shaped object in the sky that suddenly disappeared.

While driving Northbound across the Longview/Rainier bridge, I spotted a white flattened oval shaped object through the girder structure of the bridge. I hoped to myself, "Watch, it'll probably take off at a high rate of speed the minute I can see it clearly." However, once clear of the bridge structure the object was gone completely. I could not see it anywhere in the sky.
This was a definite object about the size of perhaps a medium sized plane at approximately 3,000 feet but had no flight characteristics of an aircraft designed for flight in our atmosphere. No wings, no rotors, no balloon.
It was a brilliant white but was not illuminated. I saw it in clear sun lit skies. It was also independent of the bridge. In other words not something stuck in the girders. It was an object in the sky that suddenly wasn't there anymore.
I wasn't drinking or on any drugs or anything.
But I can't explain it other than how I just did.