Nope. Only in VR. If take a snapshot of hmd + plane, then it is in the monitor. In added, that single black line, becomes TWO vertical lines (i assume one per eye). No idea what's going on
It's very strange. What comes to my mind is that you are using Crew Chief and is showing something using the overlay system. But it's pretty sure too that this idea is not useful.
And have you tried removing opencomposite and running througn SteamVR again to see if its still there?