It could be road grease too, i find the car picks this up after motorway journeys especially in the damp conditions we have been getting lately.

Good deep clean, clay and then try some glass sealant you wont look back, glass sealant is a life saver in our climate!

@[Curran] should have some in stock and guide you how best to apply it etc they are all different and have different ways of application and longevity