I was on a plane flying over europe, watching a film. glanced out of the window and thought i saw an item moving in a different direction to the background. thought it must have been something on the window, but then moved my head and the object remained constant with the background.
upon closer inspection, i could see a small vapour-like trail coming from below it as the item ascended. it hovered for a couple of seconds and then descended a bit before heading backwards (from the perspective of the plane’s direction of travel) and then disappeared below the wing of the aircraft.
checked on flightradar and the plan was around 39k feet travelling at 400 knots at the time.
i’ve taken over 200 flights and have never seen anything like it. i was so sure that it was nothing of any consequence that it took me a few seconds to fire up the camera on my galaxy s7 (even thought it was in my hand, as i was watching a film on it).