SPOILERS AHEAD (Although the description is a spoiler so...)
After you get through the puzzle door below the treasure room, you'll go up into a small upstairs room. Examine objects in the room until Zellner's investigating is interrupted by a mysterious figure.
You'll have the option to address the figure by one of 4-5 names: Dr. Gebhardt, Captain DiConti, etc. Simply select Mr. Inch, the late Baroness' butler.