sounds to me like tar from a newly paved road....it can be a PITA sometimes to get off. Reason I say this is b/c each time I have taken tar spots off it starts as a dark brown dot and as it comes off it gets lighter (dark brown, light brown, dark yellow, light yellow, and then eventually gone).
Do you recall driving on a newly resurfaced road recently?
As far as getting it off I've always had luck w/ the clay bar. Not sure what else would work well. Perhaps simple green
