Saved!
Yea those drains are pretty meaningless during a 'heavy' rain, it simply overwhelms the drains capacity. Mine were kept clean also. My pinky/finger barely makes it in a centimeter deep... so bloody small it's nearly ridiculous to call those "drains". Porsche: you are reading this?
In my case water sloshing is what everybody here is suspecting (as you've prevented, u r smarter than me man). Beyond my understanding though as I've removed pretty much all that I could find from behind the seat when it happened. However I've drove the car a day after that rain event to use my indy's industrial Shop Vac. His parking lot is awkward and to turn into it the car needs to bank something like 10~15degree on the driver's side. This is what makes them all believe I'm the one who screwed up lolll
I (personally) still blame 20 y of accumulated humidity so keep an eye on yours yearly and make sure (paramount) the car is vented properly once it gets wet.
Love your alias btw, creative LOL