I have never actually seen anyone give me the pat on the head signal and I have been riding for over 30 years. I even lived and went to High School in N.J. ( Newton, Sussex County). But yeah, I have encountered on more than one occasion drivers doing the palm down, up and down motion suggesting I slow down because theres a cop up ahead. So that is what I do now. For some reason or other I thought the tapping of the head meant in a group ride to pull over. I'm probably off base on that I guess.The 'tap the helmet' signal has been around (in NJ) for at least 40 years. It kind of went out of use for a while, but it's still the most common warning.
I've also seen riders move their left hand, palm down, in and up an down motion, meaning 'slow down'.