I headcanon that the signs are something that Lucius and Severus came up with, and it quickly caught on to the other years. To this day, Severus likely enforces it with his own students- teaching them example signs and then leaving them to think of their own signs among their circles. Maybe there's a more universal one that he teaches, one that those from other Houses would understand- some kind of wizarding sign language that branches off from BSL.

 Thank you! I really enjoy this story, so I'm glad to hear that you enjoy it so far. :D