I did watch a guide on where to find the first 12, because it the quest was time-gated, and the two areas I did was the trenchway and hall of souls, and only recently have I tried finding the last 6 for the rest of the quest, so I believe the problem one might be in the mausoleum. I have also just went back through all 18 spawns and didn't see any "something strange is nearby" except from the hall of souls but i think that was because of the sword logic explosive thrall that spawns.