Campbell's Secret Chamber is a hidden room in dishonored
There is an underneath entrance which takes you near it to a barred window, and you can hear people inside say:
"I don't see Campbell's Journal anywhere, brother. This is folly, I tell you. If he catches us in here..."
"And I've told you a hundred times, Archer, that I'm certain he keeps the black book on his person at all times. We went to the trouble of breaking into Campbell's Sanctum for nothing."
How to get to Campbell's Secret Chamber
To enter Campbell's Secret Chamber, first locate the stairs with the red carpet (near the main hall) leading up. Don't climb those stairs, instead look for the door that says "To Kennels". Go down that door and through another door, then in a dark room locate the Holger Statue and push Holger's Eye (The green eye). Once you have pushed the eye of the statue, the secret door will open and you can go inside Campbell's Secret Chamber.