Today I saw a thin, dark blue-gray streak visible in the sky in front of an aircraft—I estimate 500 meters ahead of it, and about 2 kilometers long—where that aircraft seemed to be flying in that trail. So it seemed to be following a kind of trail. Exactly, that is. It did fly under the trail, I reckon, but into it, from my position.
It wasn't a contrail-making aircraft, by the way. So, it just looked as if I saw a trail in the sky that the aircraft seemed to be flying into, and that trail once again stretched out in front of that aircraft.