I thought that too. Could it be a sunroof?
Yes, as @watfs has said, it’s a sunroof. If you look carefully, you can just see where the front edge of the sliding panel meets the rear edge of the fixed gloss black panel that sits between the sliding panel and the top edge of the windscreen.
VW changed the sunroof design on the mk8; the fixed panel no longer surrounds the sides and front of the sliding panel - the fixed panel is just a strip between the front edge of the sliding panel and top edge of the windscreen.