That is common with all vehicles, and especially easy to see w/ white paint. Clay bar will usually remove them. Usually environmental things like rail dust, driving all winter in salt environment, etc.
My body shop friend told me never to ONLY use touchless car washes, as they don't adequately remove these small iron containing environmental specs, which imbed in the clear and then corrode. Car washes w/ the brushes, or washing using high quality car soap and a mit fairly regularly are your best defense against this issue. Else a clay bar treatment once a year.
Worry not, your body panels are not rusting (esp since the liftgate is aluminum).