It's the white line before the shoulder of the road. I think he pulled a trick on me by hanging in the left lane in My blind spot, and not passing. While I was trying to find him in my left driver's mirror, to see what he was up to (it was late at night and I didn't know who it was or what they were up to), I may have inadvertently drifted to the right.
The only reason I can figure why he was checking me out was because I had a 16' extension ladder in the back of the truck - longer story...