Somewhere here on the org. I have read that VW sprays beeswax on the inner side of the metal panels.(This is prolly where the famous crayola smell comes from.
)
It's this that may be the problem. It's really sticky and would attract road dirt and brake dust to make it look like a streak.
I've had it appear on my windows in the Summer.
Maybe the temps will rule this out for you, but it's just a guess.
Oh yeah, goo gone worked for me.