I noticed lately with all the snow we've been getting that all the melted snow would gravitate towards the accelerator pedal area. I took a look in that area and pulled up the carpet and sure enough, there is heavy rust in that area.
Problem with factory BMW mats is the cutout right at the base of the pedal that allows moisture to soak into the carpet and eventually onto the metal floor.
I would advise everyone to look under the carpet area for mold or rust. I would also suggest snow people get regular rubber mats during winter and make sure it goes under the gas pedal to catch any water.