East heading away from Seattle, quarter Mile above land.

Object was jet black, cashew shaped, orientation, longer up to down. Top sw side was shining white light, likely due to sun placement but odd because the sun was already towards the west side of the object. 3:10 pm . Headed east, slightly south. I'd say from Seattle, roughly 110 degrees on a compass. Object between size of a car and a normal bus. I was driving half way on the floating bridge i90, headed west towards Seattle from mercer island about 3:10pm. About 50-70 feet from the Mt baker tunnel, it disappeared. Just gone.
I was interested and turned around by the stadiums, I took the exit, and saw in my rear view it was back and absolutely in the same area. But when I was able to turn around it was gone once again.