The event detailed in this link was not physically explained and is used as evidence on UFOs existence.
I suspect that the perceived visual effect and the radar signal were originated by a contrail.
--- facts :
Alto-cumulus clouds over Paris, wind 311º, 101 kn (187 Km/h).
Observers: pilot, copilot, steward in a plane, heading 332º (NNW), altitude 11700m, 650kmh, 21Jan94 13h14.
the path is represented in the image as yellow (the given coords were adopted as the central value of the observation).
They saw a dark object of varying geometry during 1 to 2 minutes in the direction of Paris, started at 10 oclock and ended at 8 oclock and somewhat drifting in the sky along the path parallel to the plane.
It was seen at half distance to Orly airport which has a runway at 80º exactly inline with the vision (unnoticed fact). The reported altitude of the object was 10500m
a contrail seen against a landscape can not be seen when we look perpendicular to it and is much more perceivable when seen along the longitudinal axis. So, in the start of the vision it appears to be far distant to the left and as the plane closes more and more the angle with the contrail longitudinal axis it will appear nearer.
I dont know if a comercial or militar plane had just landed at Orly Airport (ORY) leaving a contrail, but their vision seems to be consistent with a contrail.
(the linked article is in French but we can use the google translator or the Chrome/Chromium browser)
Please, feel free to move the post to a more correct sub-forum.
I'm asking the oppinion of the readers.