shelby 2015 bumper stains

[juguina]

I am detailing a white Shelby 2015 and it has some stains on the bumper below the gas cap I haven't been able to remove, so I need some help on what to do.It seems that something added to the gas tank spilled and stain the bumper and not the car paint. I have try Menzerna 400 and it doesn't do any thing.I appreciate any help
see pics.

[GSKR]

Bump up to a compound by hand.Being that it's on the plastic its very pourous,so take your time with a rubbing compound by hand with a soft micro and let your index finger work at it.

[briarpatch]

Based on the pics.....unless its a shadow or something...it looks like it may have seeped into the seam where the bumper and quarter panel meet