I was (as an airline spotter) taking some photos of an unusual visit of a lufthansa a340 at lisbon airport (usually they are a320/321) from the balcony of my house. i took several snapshots of the incoming plane doing is final approach when trough the visor of the camera spotted a very fast object passing behind the aircraft. assuming a bird was ruining my photo, didn’t care more about it. when downloading the images to the computer, and after choosing the best one (and cropped to have a better view) i spotted a dark spot. then a recall the assumed bird and zoom it. that's when i found that it was not a bird at all (i have caught a lot of them and they look like this). also appears not to be a drone, because of shape and size but specially because i used hi speed shutter 1/2000 and no propeller is visible, not even blurred. his motion was very fast and passed behind the airplane in a fraction in straight line from east to west (aprox.), over southern sky. what it is i do not know? that's why i am reporting it. will be happy to get some kind of clue or answer about this. attached you will find the cropped photo with the object (over the back of the plane) and the sequence of photos prior and after. not visible in any other because it moved very, very fast.