For anyone still doing this (Progression servers), the information below has apparently changed. It no longer has a placeholder, and instead of the circular room it now spawns in the northwest area (not the furthest northwest room, but the one directly below it). The rats are still out there and still see invis, though seemingly they do not see IVU.
Good luck, this one is proving to be a pain already. Note also this link to what is apparently the new version of the signet mob. https://everquest.allakhazam.com/db/npc.html?id=29436