Could it be some sort of fungus like Artillery Fungus or sometimes called Shotgun fungus? Parking next to fresh mulch that's infested with this stuff can get your car sprayed. It usually leaves black dots, but sometimes when you try to wash them off they leave a rusty colored spot. Very tiny dots. Shotgun fungus will also spray onto light colored houses if mulch is infested and placed close to the house. It can reach up to the second story. Seems a little early in the year for this though. Just a thought....

__________________
2007 X5 4.8i Space Gray
2001 X5 3.0 Titanium Silver