It’s not something I’ve heard of, to be honest, but there are really only a handful of ways to get wet carpet. The first is that the windscreen or side glass is leaking and allowing water in. The other chance is that there’s a rubber bung or seal missing from the floorpan that is allowing water splashed up under the car to dampen the carpet. I’ve even seen worn or damaged door seals allow rainwater to be sucked into a car on the move.
The first thing I’d check would be the condition of the door and window seals and make sure there’s nothing stuck to them. Even a tiny twig or piece of leaf-litter can break the seal and allow water in overnight.
If it’s none of those things, then you have to start looking at possibilities such as a leak from the heater matrix. This is a serious hassle as often the whole dashboard has to be removed to replace the matrix. But your car should still be under factory warranty, so I reckon it’s back to the dealership and an in-depth look at what’s going on. Don’t forget, too, that even though the water is pooling in the passenger’s footwell, it could be entering the car elsewhere.