Do you remember those black circles in the Control Room?
It was a little tricky to get good angles without getting the green triangle over the Sentinel and not going outside the circle and inside the light cone.. :)
Oh, and btw, both players (coop) were Sentinels, they have no crosshair, only grenade indicator as the Hunters.
Mouse over the images to see the Sentinels in their closed form.
BOLL created these because he was curious about how the animation really looked... after watching himself in the reflective floor in Two Betrayals (after bumping a sentinel).