When we first enter the Black Armoury and meet Ada she tells us that this place is not meant for Guardians. Now hold on just a damn second, if the Black Armoury is not meant to be visited by guardians, then why the frick is it located in the -blam!-ing tower, literally the place where guardians gather. Secondly, how the -blam!- does a golden age armory exist in the -blam!-ing tower, and we guardians, you know, the ones who own the the tower, not know about this? Like it's literally located a 2 min walk away from the Vanguard Commanders.... How the hell does Spider know about this place in the tower and we don't? How?......
In fairness it could have fallen through the cracks during all the rebuilding from the Red War. The original tower was in a different location so they have only been there for a year, trying to rebuild what the Red Legion destroyed. There are probably alot of abandoned areas in the City and Tower that aren't used, so they don't bother looking into them.