the black plastic on the spoiler and the back of the Prius is SUPER prone to scratches. I've used some Megiuar's light compound with a microfiber on mine and it took all the scratches out. Checked with my cell phone flashlight to make sure no micro scratches were left.
If you take your car thru a carwash that uses brushes, then yes they can certainly cause those scratches. Best way to avoid is to either wash it yourself with the right equipment or use a no-touch carwash (high pressure water).
