Oh, oh, I just had a light bulb moment. I hand wash this car (don't know why, previous owner wasn't so kind), but car just before this I drove through the car wash every car wash. I had brown on the back. It was worse when I got the car. Anyways, I was wondering was the brown was too, and then one day after a car wash, I dried it right there at the place, and the brown was from the car wash. I don't know if a combination of all the colorful suds made brown, or if it was something else, but I had the brown. After that I started drying it right then and there and the brown area slowly got smaller, but never fully went away. Scrubbing it would of burned the clear coat.
Anyways, if it's the same thing I had, it's from the car wash, and a professional should have no problems getting rid of it.
2014 Mustang GT Premium 401a; "Now you can add Premium to your signature." - Thereaper