Thieves' Greeting

Despite the name, it was only really used as such for a few centuries around the medieval period and then went out of favor sometime in the 1600s. A particular hand gesture used to demonstrate that one was a fellow thief and should not be stolen from, and also a way of interacting right under the nose of those you might intend to steal from.
In the early 1900s, someone discovered the gesture described in an ancient book, and decided to co-opt it for the superhero community, to have a way of subtly identifying each other.
I am not going to describe the gesture exactly, because if I did then it wouldn't be a secret. A piece of information I can give is that it can involve a handshake, but doesn't have to. Also, the finger positioning does matter, but only if you actually have those fingers. It also involves motion, so don't just claim it's a peace sign.
In the days before private social networks and radio communication (and its eventual technological successors), the thieves' greeting was the primary way that superheroes could locate each other. In more recent years, it's gone somewhat out of favor since people can just exchange numbers while on the job, or they came up in similar organizations, or they have a cousin who knows someone...A lot of options to get plugged in, and a secret handshake isn't as necessary.
I think part of why it's considered old fashioned is less the age of it, and more the fact that it's not all that special anymore. It used to be that you could only learn from someone else who knew, which forced you to carefully seek out friends and mentors. Once you learned, it was a secret signal, passed on from person to person.
Now, it's more of an institutional thing that you learn at the equivalent of superpowered preschool, not even as a hero. Anyone with powers will pick it up, as a part of whatever lessons they're taught