Touareg: 2008 R5 TDI
Join Date: Dec 2016
Second Vehicle: Still looking
Thanked 35 Times in 33 Posts
It still could be condensation. And your "problem" might be caused by that door sealing a lot better than the others, which could trap all the condensate water behind the seals, until you open it. Whereas the other doors might have worse sealing, and the water could just evaporate more easily.
Half a cup of water is virtually nothing, and the fact that the water gets released when you open the door also makes it obviously that it only stays there because it's - literally- sealed in there, not because it accumulating in either the sill or the in door itself.
If you're still worried, you can check the rubber seals at the bottom of the door and clean the openings they have at their ends, to make sure that whatever water gets into the door itself can flow out freely at the bottom.