Like I said, I saw them all the time.
Some were on the inside of the camera from something that got in when the technicians refilled the nitrogen. (Nitrogen being inert and would keep the lens from fogging up with temperature changes).
It's more pronounced on cameras such as these that can zoom in from miles away.
That would explain it following the pan of the camera and appearing at a constant distance. But what accounts for it going in and out of frame? Is it a floater like you get in your eyes?