So I have a rust issue on my driver's side door. Thought I would have a really good look around the rest of the car and I found this on both sides where the boot meets the rear light trim:
If you press down on the plastic it exposes some wear. The wear is exactly the same on both sides. My car is a 2006. Easy to miss, but mine has a touch of rust so might be worth checking yours. I think when the boot closes it rubs the plastic about.