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Also, just re-reading the intro, it appears the QM described that attempts to retrieve the blueprints or floorplan etc of the building already failed / led to a dead-end.
But how about instead
>researching the architect of the building
ie roughly when was it built, what architecture style (is it like an old neoclassical Flatiron type building, that revival style Bradbury building from Blade Runner, the Long Lines Brutalist skyscraper and NSA data collection TITANPOINTE site that inspired the CONTROL videogame or just generic modernist Bauhaus type new apartments?) Maybe it can give a clue as to the purpose
>ask about previous occupants (has no-one noticed the tunnel before?)
could either ask neighbours or try and browse / search local news for any odd happenings