representing wrotea lot of high school referees, as I have seen, will tap the top of their head to say "under a minute left". That's not in NFHS at all. I did it in a game once, while I knew it wasn't an actual signal, and my partner jokingly asked "were you telling me your head was flat"? That was the last time I used that signal and I tell others not to do that. I'm usually taking a second to look at the clock when the ball is dead.
Seriously? I've officiated in three different metro areas in two states. Some have used the head pat, others the wrist tap, and in my current association we point to the sky. I tend to chest pat if it's mine, but we always pregame the last shot as well. The reminder is good, though.
We also use the horns at 6 fouls pull the trigger finger when we have a sub for a FT shooter.