EDIT: Sorry, firenemeth, just saw you posted in one of those threads already. Leaving this here in case others need to find the threads...
There are a couple other threads on this, mostly pre-14 models, but see the quote from GlennR at the end:
Water under floor
Sloshing sound from under the car
Water under seat
GlennR had the issue fixed by his dealer:
RESULT! Water trapped in a crumple zone within the dashboard. It got in through an inedequate seam adjacent to the funnel that catches water either side of the wiper cowl. Water has been cleared and the seam resealed. Thanks to Craig and his team at Albion.
Apparently,there is a sealed plug buried high up under the dash. It was almost a dash out, but the tech managed through contortionism to get to it.
It looks a common thing that the AC condensation drains between the bodywork and the heatshield where the transmission hump is. My heatshields are saturated in that area.