Adding to comment: you don't even need any tools. Just remove the rubber stripping that secures the plastic cover and then slide out the cover. The cabin filter is pretty easy to get to. Also, the a/c high and low pressure lines are accessible if you need to recharge the a/c.

 

I read a service bulletin or something from Audi a couple of days ago, about the drains under that cover, there should be like a piece of rubber around the drain, grommet, they say to take it out because it can get clogged and cause the brake booster/battery to flood in heavy rain.. So it wouldn't be a bad idea to get rid of it in case of a freak storm...

 

It sure does clog. Floods that compartment (including the non-waterproof main computer) and then the cabin air filter, and then runs into the HVAC system and into the cabin. What a deeply stupid place to put a small drain.

 

Tried to increase size of my prior picture showing filter location & the 2 drain locations. I've read that some people have had both drains blocked & water sloshing around & getting into electronics below passenger carpet region.