It's hard to guess without seeing a picture, but I have seen something similar after a jet flew overhead. The line I saw was much darker than an ordinary contrail from a jet. But some older and/or military jets use a LOT more fuel than modern passenger jets, and they can produce much darker contrails. Fighter planes using afterburners are another example. Some of these planes fly so high that you don't see the plane itself, only the contrail. So I don't know if that's what caused the line you saw, but it might be one possibility.

I am guessing it was the shadow of a contrail, but I really can't say without more description or without a repeat of the same occurrence. What we know about science we know because of repeated observations and/or experiments, not individual happenings.