As the title asks really. I've had this in the carpet since I bought the car. A bit annoyed as I missed it when (not so) thoroughly checking the car over. But obviously saw it soon after! Private sale, so no recourse with a garage available.
Wow... I can see none of you live in Canada... lol
Looks like road salt. There is a special cleaner for removing salt deposits from carpets... But a bit of hot water, steam cleaner and a lot of effort if they are hard. Mine last year were half a cm thick in places.
I've heard of a recall on the A4's that changed the drain pipe for the bulkhead/heater system/pollen filter area because it was letting in water, and that caused the carpet to rot from underneath and the pile to then come out. Unfortunately the carpet isn't wet at all, so I don't think that's it!