If this is a changing bag issue (taking light in one way or another) you could try to use you changing bag next time with the lights on the room turned off. This way, the remaining lighting will, likely, be too low for pouring in the changing bag. And devise from here as for an explanation or a solution.
If the ghost sprocket have been closer to the edge of the image I would have thought of excessive agitation during development causing more action of developer around the sprocket holes. But it seems unlikely.