I had a Rover 75 with mystery wet boot floor, the water also ran down into the rear floor wells. I traced it to a cabin air extract vent which is behind the boot trim on the inside, and hidden by the rear bumper on the outside. It snaps into a hole in the body, but had popped out for some reason, so water could run in behind it. All I had to do was snap it back in, problem solved. If the i30 has a similar arrangement, it's another thing to check.