Does the floor really need to be 'hidden'? I used to work in an office tower where the entire fourth (I think) floor was dedicated to machinery. I don't know what kind of machinery they needed an entire floor for. I think it housed the climate control stuff, and perhaps server space. This floor didn't have windows, but there's no reason why such a floor couldn't have windows for architectural reasons. I didn't find that floor strange, and I never heard anyone else about it.
It's the second floor of the tower section, with the big grills for (presumably) the HVAC clearly visible.
Although now I'm thinking about it, it might have been just this hidden society floor, of course. This was a bank, so that would be the secret meetings of the elite to control society. Wait, that isn't actually secret...
Thinking some more, do you actually need a hidden floor? Any self resprecting big company has some top floors for the directors only, where the underlings are not allowed to come. Nobody questions that. Any self respecting secret organization will have one or more front companies readily available.
edit: I should've just wikipedia'd it of course. That page offers many ways to hide secret floors in plain sight:
- "As a rule of thumb, skyscrapers require a mechanical floor for every 10 tenant floors (10%) although this percentage can vary widely", so just vary that percentage, and use some extra "mechanical floors" for secret meetings.
- Some types of buildings require outrigger trusses connecting the building core to the external shell at regular intervals for structural stability. If those are more than what's actually needed for mechanical floor space, there's your secret society floor.
- Sometimes the mechanical floors are clustered together. If you pair the mechanical floors and give them a lower ceiling height, you can add a third floor, while making it look on the outside as if the mechanical floors have an extra high ceiling height. Apparently that's just normal.