I got the car a few weeks back and noticed when i got it that the boot was about a quarter full with water. Just a few cm under the ecu ( or whatever that box is in the boot). I noticed the spoiler was not tight at all on the left hand side and nuts were rusty. So i thought it will be an easy fix. Got home with the car, striped the boot, got a water hoover at it, got it all dry. Removed the spoiler, cleaned the old rubber seals on it and the surface were it was mounted and tightened it properly. All ok till this morning when i noticed again thecarpet is wet. It's not soaked like before, but it's all wet. Tried to source a leek all around the seal but all seems dry.

If anyone came across this problem please HELP!!!

Check the inner guards behind the boot interior linings.

is top part of wall between engine bay and boot ok? rust, holes or anything else that might let water in via engine bay?

Im actually having the same problem now, i cleaned everything out and the next day it was full of water again, surprisingly there is no rust though.
It only happened after i took my rev3 spoiler off and plugged the holes with little rubber plugs, i think its somehow getting in through the holes for the spoiler, maybe silicone would help.
Im gonna fill them holes and wrap the boot lid in carbon, i hope that will sort out the problem.

There is a bit of surface rust on each side, in those small poket like holes. I had to squeze the pipe from the hoover cause they're very tight. But at the moment they are dry. I'll see in the morning.

Hope the rainy season stops soon. Fingers crossed!

Check the seals on the rear lights and arial.
Is your boot closing down far enough
And check the seals around the wiring goes though the bulk head can leak badly

This also happened me and ended up been off the road for 6 months due to my ECU going faulty, sort it as soon as you can, can be difficult finding an exact ECU match for your car