Seemingly ages ago I had cleared out the Mausoleum (or so I thought)... then I decided to go back and grind a bit and get some loot (the platemail drops that some of them give add up quickly) and lo and behold, I found the secret door leading to the Lich and the Myrosian Gauntlets! A nice surprise... makes me wonder if I missed any other secrets from places I cleared out early on but haven't gone back to...