The exposure time was 600ms, or 6 tenths of a second. That means its just over 1/2 second. About as long as it takes you to say "Eleven". So although I suppose its long enough for something to move through the field, I would think it would really have to be moving fast.
Now, I'm not seeing the dark area in any of the reflections. So that would lead me to believe it was not someone there at the time of the photographing.
The one thing that bothers me, is if there was a shadow person such as one pictured, you would certainly see some evidence while you were taking the picture. Now its possible it happened while the photographer was not paying attention. But I just find that a bit odd. As I've said before about photographic evidence, generally if you're going to see something in a photograph I would expect that it was also visible during the time of the photographing.