The disappearance of planes and Unidentified Flying Objects have a long history together. Many boats and planes which went missing in the Bermuda Triangle are supposedly related to UFOs. This is a small following among theories which state the some well known plane disasters are related to UFOs in some way.
This seems true with the recent EgyptAir plane disappearance. Two Turkish pilots reported "an unidentified object with green lights passed 2 to 3,000 feet above..." when they were 17,000 feet up.
The encounter occurred just an hour before the EgyptAir event.
You can read more details on the crash and the UFOs here.