So let me get this straight: The Imperial Palace alone, aka the place were only the Emperor and his Custodians are allowed (and maybe some inquisition) is covering all of fucking europe. So best case scenario 10-100.000 people are inhabiting all of europe...it would be so easy to sneak in, because even if they did their best they would need at least an hour to scramble to that position if you get found.
That aside, i imagine it being full of unused rooms, filled with all kinds of shit which would be useful to the Imperium at large, but nobody bothers to actually write down whats there and how it could be put to use.
As in " Oh, thats the STC-Room, we have like 50 of them or so lying about so we put them all in there...What do you mean you were looking for those the last ten thousand years?"
"Oh that? That is the Titan Garage. We occasionally use them as pillars when one of the pure gold ones collapse during our trainings."
"This? This is the Terran Library, we have in here every book ever written, in fact it also has access to the Black Library, but nobody ever bothered figuring out where exactly that access gate is so we mostly just close it off."
"Yeah, yeah Emperors Laboratory, and yes there is two primarchs in those tubes. But nobody said we should wake them up so we left them like the Emps left it."
"Oh yes this is the Manual to the Golden Throne...but none of us can read this so called Swerish? Swelish? Swedish? Who cares, also the pictograms are way to complex, if you ever find this I-Koa or whatever you should ask him what stands in there."