Did you buy it new from a reputable dealer? If so: likely not refurbished. These cards can come from the factory looking "washed." They often clean them in something like "2B denatured" ethyl alcohol etc. From there you might see dust from packaging materials. Remember that the boards are often shipped in build shipping containers before packaging so they may pick up stuff from that.
Not a reputable dealer, I've already got other refurbished stuff from them earlier, that in the end made them to give me back my money.
Anyway, that is not dust. I have cleaned a lot of things with cotton and ethyl alcohol in my life, so I know what left-over cotton parts look like. It's visible in one of the pics, by the way, but I can upload more pics of exactly that cotton.
It's a few "slices" of cotton, stuck to the board after cleaning.
EDIT: At the moment, I do not have a way of testing it.