Sorry if this has been covered already.
That looks very much like a blend line. Its where a paint shop hasn't painted the whole panel and stopped. The edge where the colour or lacquer finishes is dissolved with a blending thinner. Later its polished and the edge disappears. Problem is, home polishing/waxing can pick the edge up again. A bodyshop may be able to machine polish this away again but it doesn't' always work.
Jon