It's fake. Presenting emotions that you do not feel to affect the emotions of others, even if it's to brighten their day, is disingenuous. Just get the "Mainer Nod" down, little reverse nod raise of the chin. Acknowledges that they are there respectfully without indulging in some small talk song and dance.

Never heard of it.

That's just New England, Maine in particular. You are under no obligation to smile, make small talk or blow smoke up each others asses with stupid chatter while in line waiting for something. I love it, but I hate that fake shit so I am for sure biased.