spiders have paint stripper in their webs.
haha^^
I get this problem every once and a while, but i live out in the country so its expected. check your engine bay and little places that are protected from the elements. in my old car, there was a spider that made a nest right behind my battery and i had webs all over the car pretty often. killed the spider, cleaned it out and didnt have the problem again till i moved out to the country.