I have exactly the same problem with my 2014 F11, except my floor mat was soaked. I've had the car back to the dealer who kept it for 4 days over the weekend and apparently stripped the car back and water tested it twice but couldn't find where the water was coming from. I've since collected it and driven it for a week. Floor seemed to be drier (but still damp) but has been wetter again, although not to the same extent, since the rain at the weekend. I've read elsewhere that there is a common issue with the vapour barriers inside the door cards and the water effectively runs out of the bottom of the door card, and hence in to the footwell, rather than being directed through the drains around the edges of the door to the bottom corner where it drains. My car needs a service in a couple of weeks so I'm proposing to monitor until then but I'm not convinced that its a one off and the issue has been resolved.