Very fast object photographed over Lisbon, Portugal

On September 15, 2017, a witness has caught a picture of a very fast UFO over Lisbon, Portugal. He has only detected it on photography later.

Witness statement:

Doing some planespotting from my window, I notice a very fast "thing" passing through the corner of my eyes as I looked trough the camera. Only later, when was downloading the pictures I noticed this strange object. First I thought to be a plastic bag blown by the wind, but then looking better its strangely symmetrical and much complex. It only appears in one photo from a multiple shot sequence.