I posted about this exact same thing yesterday.
My rear wheels seem to get covered in brake dust really quickly, sometimes it appears quicker than the fronts, but they do also collect dirt more too.
I was always under the impression the rear brakes hardly did anything, which is why I find it puzzling/slightly worrying!
I fitted replacement standard discs and pads a few months back too. Took them off yesterday, but couldn't spot anything untoward! Although to be fair I wasn't really sure what I was looking for haha.