Just flew over Turkey somewhere over Izmir and Khios Agean sea, heading into Athens around 4.30pm today we'd 21 March 2018. It appears to be flying away at rightangles from our commercial jet and somehow without either of us appearing to turn we end up almost following it.
Looking back on flight tracking software I would suggest that the Qatar Airways flight was QTR211 flying at 35,975 feet.
The crossing airliner traffic I believe would have been the higher of the two airliners to the north.
TUI Airways/BY2337/TOM41W flying at 34,000 feet.
Playback of airliner identified as "UFO" in the video. QTR211 is aircraft flying right to left in the playback.
The airliner following the TUI Airways/BY2337/TOM41W was approximately 4,000 feet lower at 29,975 feet.
A lot of the confusion surrounding this video is the dark coloured contrail. It tends to confuse most viewers who suggest that is either a missile or a military jet fighter.
Contrails depending on the lighting conditions can be varied. This can be seen in the following video. See debunk of that video at following link.
Of note on the "Smokers" video is footage of two examples of crossing airliner traffic similar to what is captured in "UFO/Turkey" video.
See from 06:41 in the "Smokers" video
See from 04:05 in the "Smokers" video. (This airliner is obviously closer/longer contrail)