I see at least a small amount of this in almost all Kirbys I work on, in most its just a little bit that scrubs right out when I wash the parts, but some have much thicker layers of it. I agree with Tom, its most likely food particles, even food thats not as greasy will still be more sticky than most types of dirt.
I've also seen sometimes a very sticky black residue on bottom plates and bruhrolls, its much nastier to get off than the food stuff, and I don't know where this nastiness comes form.