As it became clear that a larger metagame was taking shape, fans congregated at places like Game Detectives, a subreddit that’s become famous for decoding such puzzles. Through careful experimentation and data mining (a process through which players examine a game’s code), diehards have discovered 19 sigils. The most recent one was found in You Have to Win the Game, a retro platformer released back in 2012. The sigil was presumably added through a patch at some point, but no one’s quite sure, really.
Here are the collected sigils:
I love when stuff like this bubbles up from the depths of the internet.