Had this issue for a long time. Located the source. Water is pooling in a seam between the two skins of the rear of the boot/trunk. Once there's enough water in there, it overflows through a hole punched into the inner skin, just above one of the air vent flaps.

I have yet to find out how the water gets into this space (never happens after washing, only happens after windy wet weather).

Someone mentioned that there is a seam that may not be sealed properly, behind the bumper. So I took off the bumper.

I found a small area that seems not to be sealed, just above the cable loom entry grommet.

Just wondering if anyone else has this issue and has fixed it. I'm going to seal this point and see what happens.

(spolier drains are fine)

Unsealed area behine bumper.

hole where water flows out of.

 

can you make a zoom-out the picture where you show the entry groomlet? maybe will get some idea how the water can go there... and what is the relative position of this groomlet vs the hole you say the watter goes in.

 

Here it is. Any water running off the back of the car runs down the channel at the side of the boot/trunk and directly past the gap in the selant. I think the factory don't seal this location due to the proximity of the cable grommet. There is a very tiny gap there and a combination of capilary action and suction allows water to get pumped into the gap between the panels and eventually into the wheel well.