I had this on my old mk8, mentioned it to the dealer when it went for service, showed a picture to the tech of the water on the plastic sill in the rear and he felt that the rear mat was damp, they agreed that water was Present, when I picked the car up, they said they’d water tested the car for 24hrs and had found no ingress, I took the car home and in the next 15000 miles until I traded the car it never happened again?! I’m not sure if they messed with something or not when they had it, but it remained dry. 🤷♂️