There are different customs in different parts of the world. In the South, that is the southern United States, we nod and smile when passing someone in a store, on a sidewalk, in a parking lot, or in a hallway. Visitors or transplants from other areas have difficulty at first wondering if they know the person and are just being rude having forgotten their name. But no, it’s just our way.
Why do I nod and smile, or at least smile? For one thing, I’m a Southern Gentleman, and for another I am acknowledging that upon making eye contact that you are a human being of worth and that I have no ill will toward you. I don’t know where you are from, your history, or anything about you, but for that moment we are two equals who share a clean slate with one another. It’s a greeting and agreement rolled into one meaning that I mean you no harm and I expect to receive the same treatment in return, not the smile and nod.
So if you are ever down South as they say and someone nods and smiles, then nod and smile back and venture onward. You didn’t forget their name and 99 times out of 100 they aren’t flirting…maybe.