They would have had very good reasons to completely scan the entire facility, every inch of it top to bottom. Ultrasound scans of the walls would be a basic requirement to discover hidden passageways. The Fallen have cloaking tech, the Cabal were everywhere. No chance could be taken on hidden enemy forces or hidden bombs left waiting on a Vanguard force taking up residence. Having everything magically hidden behind a disguised door is just silly.
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But how do you know the vanguard posses this kind of technology, or that the black armory doesn’t have some sort of defense to it?
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Why are you so sure that the Vanguard wouldn’t have access to such a basic technology as ultrasound (never mind a futuristic improved version). Why do you believe that, following the events of the red war that the Vanguard would end up in the same tower as the black armory ? That in addition to such a coincidence the Vanguard would fail to secure their new home ? Why do you not consider the fact that the black armory would have detected the first Vanguard recon team on approach to check the tower and would have made every effort to ensure it wasn’t suited as their new home ?