Fallout, it really shows up on white 500s,
You get it from the pollution and tiny specks as the brake discs wear. It can be fallout from the railway if it was transported from the factory by rail (high end cars have "pyjamas" to reduce it)
Remove with Iron out, or, I use Valet Pro Dragons Breath (also on the wheel)s every year or so.
Then clay, polish and seal (wax)