Disc was flying about 50 yards off the wingtip of an airliner that had just left Seatac airport.

I had just come up out of a dry dock and was walking back to my reporting area, when I heard an airplane overhead.  I glanced up and saw the airliner that had just taken off from Seatac airport, as it was at a low altitude , about 2500 to 3000 feet.  I looked back down, watching where I was walking, but had noted something odd about the airliner.  Something was flying by it.  My first thought was that there were pigeons, or seagulls flying in the air.  I stoped walking and looked up again at the airliner.  Then I saw a round disc flying about 50 yards off the airliners port wing tip.  The pilot must have seen the disc about the same time, as the airliner made a sudden turn toward the starberd side, changing its course to a north direction.  It had been flying in a westerly direction.  A moment after the airliner turned, the disc also turned in the same direction.  However, as the disc made its turn the rear of the disc dipped down and for a moment I could see the dome in the middle of the disc.  After finishing its turn the disc leveled out and cought up to the airliner.  Both continued flying in a northerely direction.
Date is approximate.  PD