Im having a problem with that quest as well. I have found all four entries, but i cant get to the chest. I think im supposed to inspect the portrait on the second floor, but it doesnt appear to be able to be activated. Not sure what to do...
Yeah, holding the tab button alot is really important in this game if you want to find everything. I hold it about 90% of the time I'm playing. About the portrait; yeah, you have to right-click it, then select recite the warden oath and a chest will appear.