Went for a car wash last night, and noticed this on my roof when I got back. It does not seem to want to come off. It almost disappears when wet, but then comes back when dry. Maybe ScottWax can shed some light.. I live in Toronto, so I can't use any products that require warm temp. 80% of the time the car is sitting in my underground garage (condo) as I walk to work. I am guessing it is a water stain or bird droppings.