The other day I was watching the trails being laid in the sky and noticed a dark line extending from the horizon and surrounding the trail being formed. This a lot like the one I saw (I did not take any of these pictures):
Note the line continues past the trail meaning it can't be a shadow.
These are similar as well:
I have seen cases where the plane vanished mid-flight, like in this photo.
Apparently the line can form even without a trail: