Following on from my rust thread, could you all do a quick check when you have a few minutes. As I posted in my rust thread, my A3 had a new passenger door last year due to an accident, and it seems to be a modified design as it differs from my original 2005 drivers door.
If you could check your door design (if it is original) and if you have the new design post the date of your car as it will be interesting to see when the change came in. If you look at my picture below you can see that the door has a cut out to allow moisture to drain from behind the plastic trim that is at the bottom of the door. My drivers side doesn't have this cut out.
Modified door picture :