Even when the Instructor is absent they still gitcha! My very first formation solo flight I was at the Navy's perscribed 10' aft, 10' below and 45° angle when the Instructor (in a 'chase' plane) keys his mic and calmly says to me your mixture should be at lean position. A glance to my left and sure enough mixture full forward.
How the holy he## can he know that I think to myself.
Heads up and do the scan thingy ... no not on my wing hmmm?? Casually I look up to see he who must be obeyed inverted 10' over my cockpit looking at me with a bodacious smile on his face.