Ken, I noticed something similar on my 2003 4/4.
There had always been a small paint run at that place, but it started to develop, into what you describe.
I carefully cut the paint away to find a small area of oxidation.
I then rubbed this down, primed with Isopon Zinc paint, before rubbing down and finally painting.
Because the are was so small, I brush painted the top coat, before polishing, initially using a fine cutting paste.
Best to treat it before it spreads further.
