Lost or stolen guns will be in demand by the bad guy. You could code this hypothetical signal uniquely to each gun and shut the signal down when the gun was reported stolen.
But two officers draw on a guy with a gun - and the “friendly” signal goes off as they signal each other - do they hesitate?
Or Edwards’s gun would have said “friendly” and the officers would have known friendly was the partner next to them and not realized it was Edwards’s signal.
This works great when the cops know the guns are missing, which isn’t the case when they’re left in the men’s room.