If you have HOW press the ALT key and it will show up in the same way a door does.
If you don't, move your cursor around on the 3 pillars in the middle of the room by where the skeleton mage was. It is on the pillar nearest the door you came into the room.
It may be of some comfort to know that the first time I played it took me ages to find the damn thing! It's not too obvious.
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Time is an illusion. Lunchtime doubly so.
Epona of The Laughing Hyenas and The Ladies Guild.