I'm a little hesitant to jump in on a conversation that seems to me to be going in some pretty wild directions. But I just can't help myself.
I'm going to predict that this is the 5D Mk III. Why?
Here is what Stephen Oachs said:
I'm currently on a shoot in Kenya, Africa, and I ran into a Japanese man sporting some gear I didn't recognize. While his English was very limited, I was able to confirm that he works for Canon and is doing in-field testing with the new Canon 200-400mm with built-in teleconverter. He also was testing the new Canon 600mm.
My thought: I don't believe Canon would send a photographer out to do field testing in a public location using a body that is not close to being released. The absence of the on-camera flash probably rules out the 7DII. The grip is clearly an add-on. So, my money is on the new 5D III.
As far as this being some sort of conscious effort on Canon's part to "leak" something, I'm doubting it. I'll leave that to the conspiracy theorists. But, I suspect that the Canon employee was instructed to tell anyone who noticed that he was field testing the lenses. No trade secret there. The 200-400 was announced a year ago. I'm guessing that if he were asked about the camera, he'd simply pretend not to understand or give some other, non-committal answer.
Not nearly as fun as speculating about some grand, secret strategy. But, when simple, obvious answers are available, it's usually best to go with them.