I noticed this too just last week.  The van had been parked facing the
sun in the deserts of California for 5 days.  I'd used a sunshield
(reflecto panel thing).  I can imagine that the heat became pretty
extreme.  Mine was a white powder covering all internal surfaces of
the rubber only:  upper, lower, right, left.  I just scrubbed it off
and oiled up the rubber with my favorite armor-all substance.  Sure
left me wondering what the heck that was!  What WAS that?

-Ryan (happy to be back at the coast where it's nice and cool)