Deb, thanks for that link; it was informative.
It says adults are less susceptible than pups & weanlings to the bacterial infection, which goes along with that I have so far only seen it in my grow up cages. However, if it is the bacterial infection, I should also be seeing other symptoms such as diarrhea, which I'm not.
Further down the page it also mentions pinworms, which seems to be the other likely cause.
So I still have no idea what is causing it. I think I'll just have to take the next one to the vet and see what they say.