Every Centurion I've killed have carried ebony when I've killed them. My guess, either it's for a quest and you can't take it till you are supposed to or, far more likely. Bethesda cut them out and forgot to remove that one completely.
The spheres shoot arrows if you're a certain distance away, so I guess these are the arrows that they shoot. Try heading to one, and try to get it to use it's bow that it has on it, and see if it used this arrow.