I used to wave, just a left hand straight up an inch or so off the grip. Most of the responses tended to be strangled. I think they are confused. I ride a red BMW 650 with knobby tires. I wear a yellow helmet and black jacket and brown pants and EMT boots.
Since I don't have a "real" BMW, the Beemer crowd is confused. Cruiser guys usually give the down low wave but I'm not sure if they are flicking their butts or stretching or waving, since they rarely make eye contact. Big tourers wave or adjust the XM antenna, I can't tell. Connies, STs, FJRs- that crowd are the constant wavers. They also are the ones that will turn around and ask if I am okay when really I'm waiting for traffic to thin so I can pee. Or take a picture. Or both.
And of course, women riders wave at everyone.
There is also confusion since I don't wear "the uniform". No one piece Aerostich. No chaps and vest. I'm poor. And cheap. A lethal combination. I wear what I have and spend my money on important things. Like whores and cocaine.
The only riders that see me, and get it, are my fellow CARs*, the KLR guys. They smile and wave enthusiastically.
These days, I care less about other riders waving and more about awful traffic so I do the turn and nod. But I will return a wave if I get one.
*CAR- cheapass rider