Was once hanging out with my wife in the lobby of an Israeli-owned hotel in Costa Rica (lots of Hebrew and Spanish with some English). Two young female French backpackers stopped in for about ½ hr, sat nearby and started speaking French to each other. My French was quite rusty but it became clear at some point that I was the topic. Eventually they got up, and one, without really looking, said "au revoir, monsieur". To this day I regret not replying in kind just to see the reaction on their faces.

Over the past year my wife and I have learned Esperanto to near-fluency. However contrary to the goals of the language, it is a quite effective "secret language" when we don't wish to be understood.