Thief Side Mission: A Lady's Bequest

There are guards paroling the area, so either work fast as you pick the lock, or take them out and be leisurely about it. Loot everything that you see, and head downstairs. Loot everything... I think you get the idea by now... and crotch down to enter a duct.

Inside the hidden room is a barrel with a switch disguised as a cork. Push it to open a secret room inside the secret room. There's some more stuff down there, but it isn't where the quest item is. There is, however, another ugly painting to steal.

Head back up and through the duct, to where there's a small, round grate on the floor. IGNORE THE GRATE. Garrett couldn't fit down there no matter how hard he tried. Instead of trying to mess with the grate, turn around, and look for an old bureau that you can climb. There's a small space between it and the stairs, that has another bureau tucked away in it. Grab Lady Christina's Will, and your mission is over.