I've had a brother give me the MM sign subtly without anyone around us nothing. I went to a restaurant, and he was a server. (Stranger, I'd never met him before.) He noticed my ring, introduced himself, and asked if I was finding everything 'on the level tonight' while he quickly gave the sign of a MM. No one noticed it or took it as anything unusual, partially because they were wondering what he meant by on the level, lol.
But anyway, it was a good experience, as the both of us, complete strangers, had connected in a fraternal way in just a brief passing moment. No one realized that he gave the sign, because they didn't know what to look for.
Of course, that did lead to the, "I'm a Freemason" discussion with my friends, who were oblivious. One of them had never even heard of Freemasonry. So, him popping the phrase 'on the level' introduced a group of people my age to Freemasonry, as well, since it sparked a discussion.
Just an anecdote.