[coldtea]

>what body language tells it that the guy is about to throw a piece of rock or a bone when it can't SEE the object?

Tons of things, from being able to understand body cues that the guy is up to some BS (like we can tell a liar), to smelling the thing he has in his hands...

Plus who said it "couldn't see the thing" in the first place? Even if it couldn't see what in his hand, it could have watched his moves before, as he left the apple and picked up the rock etc.