Camera Lens Flare from the high beam reflection against the speed limit sign at the side of the road adjacent to where the “ghost” appears. Happens all the time with very bright objects. Commonly seen when you photograph a sunset; another “ghost” sun will appear somewhere else on the image depending on the angle of the bright source to the lens.
That is a good explanation except that likely the only reason he went back to look through dash cam video is that he saw the ghost first with his own eyes. I agree, the video itself is a nothing burger and doesn’t prove anything.
Did that look like a reflection of a sign? In your own example, the sun, the reflection is another sun. So in this case, the sign appears as a human standing there and not a sign?