Came out of work yesterday and the car was covered in brown stuff looks like sand , so I jet washed it last night topped it up with SN. Came out drive to work and covered again, is it sand? 
Had this same thing this morning, and like you jetted off and applied another coat of SN... It does look like it was sand, it was definately not the usual crap, and it was a sand colour...
It wasn't just my car tho it was pretty much all of the cars in my street and I saw plenty of peeps driving around with the stuff on there cars..
GRRRRRRR!