Water leaks can be very difficult to track down. For example it could be coming in from the windscreen, into the carpet under the dash and seeping through. The backing of the mats is likely waterproof to some extent so won't be seeping up through that immediately.
With the wife's polo, water was dripping through from the top of the door, onto the back of the drivers seat, eventually soaked through and was dripping from the bottom of the seat onto the floor in an area under the seat. Couldn't understand how it was steaming up at every opportunity but nothing seemed damp. Sussed it eventually.
One tip I came across but never used was to put different coloured chalk around suspect areas like the windsreen, door seals etc and see what colour the water was inside the car!
