Balloons?
On the other hand, it was remarkably like something three of observed from the office of a particular three engine aircraft (known as "The Gripper") en route LHR-FBU many years ago. It crept up on us over Drammen, (day, VMC) 4000 ft, high speed descending, took up a position just off our starboard wing for a few minutes, then sheared off to the East at very high speed. It looked slightly larger than a cricket ball.
Prober