Worked construction some years back on a tower for an oil company.  So, one day near the end of the project I'm told theres some work in the ceo office.  I ask where it is since everything is finished.    Then the boss walks in, flips a switch on the wall and the wall opens in a way that you would never see it if you did not know what you were looking for, and it led to a room with floor to ceiling white tile and a drain in the middle.  It had a second door to the hallway that I walked past without ever noticing. Must be a common trait.