Lexa glances at the altar and rolls her eyes dismissively, this doesn't appear to be the real one.
Towards the back of the room is a very plain looking door, almost as if no one wanted to call attention to it.
Zieffar recalls that the followers of Oghma had a tendency to hide artifacts of great knowledge within His altars both to honor Him and to keep the objects of knowledge from falling into the hands of the common people. Perhaps that is what you are looking for?