It seems to me it would be pretty easy, especially in some buildings, to hide secret passages and even whole rooms, behind walls that nobody questions. If you work in an office, think about the parts of your floor where you don't go, and the passages and turns that make it difficult to be sure whether your lift shafts are actually flush against this or that wall. How much of the total floor area is accessible to you, and how much of it could easily house another whole room like a board meeting room?
Mokalus of Borg
PS - It would be an ideal place for the Shadow Board of Directors to meet.
PPS - Though what could they really do from such a secret location?