Looking around the second floor, Victor discovers a few things of note in his intensive search (as does anyone else who gets involved).
Firstly, the statue in the acolytes quarters is not actually a statue. Close examination, and your own wizardly expertise, reveals that it is actually a cultist transformed at some point into stone, presumably via a petrifaction spell (there are not enough like him to account for something like a marauding Medusa).
Secondly, there is an old scrap of paper in one of the guard rooms, partially singed and with several of the words obscured. The rest is in Abyssal.
SpoilerHave to get out... stairs a death trap... get to throne room... Yah...
Thirdly, one of the pillars in the central shrine is hollow. There appears to be no way inside up here, but Victor's engineering and architectural knowledge suggests that there might be something on the floor below. Failing that, it also overlaps with the odd section of man-wrought stone in the caverns below, specifically the mud pit chamber.
Finally, your methodical search brings you closer to the room in the acolytes wing which you heard the movement coming from. You can definitely hear it now - an odd sort of liquid sound, perhaps from an ooze of some description, coming from the chamber in the very north-west of the floor.
This all takes you something approaching two hours, assuming the four of you divide up the area equally and search at the same time.