3 taps of the left palm (or 3 fingers) to the helmet is a widely-used signal that motorcyclists use to indicate that police are in the path ahead.
Equivalent to flashing your high beams at oncoming traffic to warn of a speed trap ahead of them.
Why signal this warning at that time and that place???
Excuse me, but from everything I've been able to find, and it has been quite a lot (I've invested many, many hours reading and digging outside of FR), ONLY A BLIND MORON BIKER would have needed a signal to tell him cops were present. There were plainly marked police cars all over the place, according to the cops themselves!