2017 A5/S5 After carrying a passenger today who said they could hear water sloshing around I checked the doors but these were all dry. After looking online I then checked the sills and both sides had a substantial amount of water in! I’d guess more than 5 pints. So I’m now leaving both rear plugs out. I found plugs at front and back of the sills close to the wheels. May be worth checking for others too! 

Thank you for this valuable info. I knew q7 could suffer from this but not A5 models. 

I have had similar on my 2018 A5. Heard the sloshing around, looked on this forum, and found this thread.

Had a look under the car on Saturday, and there are 5 plastic "plugs" on each side of the car. Started on the passenger side (where the sloshing noise was coming from) at the front - First 3 all had 1-2 drops of water come out, nothing special... Last 2 (the back 2 under the rear door) both emptied about a pint of water each.

Nothing at all on the driver side from any plug...

I've got assume this is a blockage somewhere, since the driver side was completely empty in all compartments, so does anyone have any ideas where the water is coming from, and what I'd need to do to remove the clog?

Thanks in advance...