Mission 16: The Plan | Secret Doors DMC: Devil May Cry Guide

DOOR 1/2 (required Gold Key; bonus mission Extreme Traversal) - Door is at the very bottom on the staircase, to which you can get during searching a 106th floor. You need to go through a well hid passage and fly over an abyss earlier.

DOOR 2/2 (required Copper Key; bonus mission Bait and Switch) - Door is on the 154th floor. It is next to the elevator which takes you on 159th floor.