The inner workings of the lock mechanism at the back of the vehicle (trunk?) begin rusting not long after the car is made. Moisture that collects inside this mechanism will drip onto the back of the vehicle, a few inches below the lock. Over time, this stains the paint with rust. You can remove the stain with regular application of cleaning or buffing compound. This is commonly referred to as a "s*** stain" on the PT's "ass." Very embarrassing, and I've never seen such a thing on any other vehicle ever made.