Like so many others of you, I don't advertise my hobby but neither don't conceal it.
Not being a fan of funny apparel with slogans helps the first part, of course.
A long time ago I had my proselytizing phase where I preached the gospel to a lot of people. I learned that many people just "don't get it" and turn down any offer to just try it. Well, their problem, they are missing the fun!
Working for a software development company, a lot of colleagues have at least a passing acquaintance with RPG and/or have formerly played. If the topic comes up I don't have a problem to discuss gaming regardless of who might hear it.
A year ago one of our testers, who had learned from my hobby when I was discussing our last sessions with to colleagues who are playing in my group, asked me about it and expressed interest in giving RPGs a try.
I offered to run a demo game for the testers if he could garner enough interest from the group. The result was disappointing: of the 14 people, one wanted to try it, two gave a lukewarm "yeah, why not", one turned down the offer because RPG take too much time, and the other ten people weren't interested at all. And we are talking about university students, a group which produced a lot of gamers 20 years ago. Sigh.
So I stay on my island of gamer happiness and let the rest of the world wallow in the mire of the uninitiated.