I've seen these light brown spots on my vehicles too and they
seem to appear when the pollen counts are high in my area.
I've seen this stuff accumulate on a freshly washed vehicle even.
I'm sure it's on everything, but shows up best on a shiny car's paint.
I don't have a garage, so all I've been able to do is keep my
vehicles super slick so it blows off when I drive, and also be very
diligent about my daily california duster and QD treatment.
The spots after a rain shower are probably mostly dust from the
air as well as some pollen too. The pollen spots are usually
smaller diameter and have a small amount of height to them.
The dust spots are flatter and larger diameter. I don't let them
last long enough to take pics of them otherwise I'd post some
to help determine what it is we're all talking about. Just my .02
cents worth...hope it helps.
