Just noticed it today, coming home from a drive. Wasn't especially humid out. Mid '60s. Sunny. It hasn't gone away after a few hours.
I don't see any water droplets or anything. I can't tell if it's condensation or maybe some soap from washing car got in there?

 

I believe it’s common in these cars. It would happen to me after some drives. Always evaporates. It could be due to heat coming from the rear, outside temps, etc. Like everyone wish it didn’t happen but learned to live with it and ignore it. Nice car!

 

Is that what it is in my photo though? After more than 3 years, this is the first time I've ever seen it.

 

It looks identical to what I would see. I never timed how long it took to disappear. As you know these cars take a while to cool down. I think it’s lingering longer based on the engine and outside temps.

You can try a test next time it happens. Keep the rear lid open and see if that helps.

 

Normal, working as intended. Same with the headlights.

Unless it turns into a pseudo-fish tank, wouldn't worry about it.

 

Normal

 

have same in my RS (left side too)

 

Like... permanently? Or condensation like that forms every once in a while?

 

YES. Perfectly normal. Condensation.
Happens with mine regularly after washing thoroughly and also under certain weather conditions.
Same with my Audi and other previous VAG cars.

 

every once and a while, it usually dissipates.