Darren and Jenny. It is called a variety of things, I have always known of it as filter dust or filtration soiling. Anyway it is not caused by air bourne dust, quite the opposite actually. Hot air rises as we know and what it is is the dust from under the floor boards or in the walls or wherever it may be being drawn through the carpet to the warmer room, therefore the name filter as the textile is acting as a filter. This you will sometimes see around the edges where a kicker or power stretcher has been used, also sometimes just in the middle of a room for no apparent reason till you lift the carpet and find a hole of some sort. Blocks of flats and staircases are the worse for this type of soiling.... Anyway you are quite right in the way to remedy the situation but beware it is a pain in the bum to fix and without sealing the "draught" it will always come back. Not only that but as you apply suction to the soil you actually make things worse or can do as you are just pulling more soil out from the cavities so to speak, hope this helps, best, Dave.