Layman friendly tl;dr: On the Promethean Code mission there is a screen[2] that you can scan with your ghost.
That screen contains a large number of cryptographic hashes[3] which are commonly used to encode passwords or passphrases. This specific screen was crystal clear while most other screens in the area are somewhat blurry. The screen contained a hint: the secret phrase used to generate the hashes is one line, three words, a total of 24 bytes.
It took almost two weeks of trying different combinations to finally crack the secret.
You can verify the results here: https://paulschou.com/tools/xlate/[4]
Secret phrase (including the period):
Hash code from in game terminal:
So this is most likely a reference to the Seraphim Vault. Which is the name of the vault where we all fight Omni'gul when she is attempting to corrupt Rasputin.
The Vault, if you look into it now, is very different than it was before. They have added in some pillars, no idea what that means or why.
Apparently from glitching you can also get into that room, and there is a room underneath of it. No idea what it is for though.
Also, and this is pure conjecture:
It is rumored that the Seven Seraphim built the Warminds. There are references to the Seraph, the Seven, etc... all over the place (Shaders, Gear, and a ship)
Could this be something that will be brought in later with the Cabal Expansion?
Will the Seven come back? Will Rasputin finally get off his butt and help the city?