I was told by the mage's guild to go slay a mage at the Lysausa Coven. As far as I can tell, I've been in every room, every hall, clicked on every wall hanging, every nook and crany. I can't hear any creatures yelling no matter where I go. I went back to the main entry room and suddenly a mage zapped me with something. I attacked and killed the no-name mage. I didn't get a note that I had finished my task. As far as I can remember, this is the only "mage" in the entire dungeon. I saved. Then went back to the mage's guild. I got a promotion but the quest giver won't talk to me now. The quest is still in my log. The time period hasn't expired. Have I missed a super secret mage somewhere? Will he/she look different than your average mage? Do you always get a message that says you have finished your task and to go back for your reward?

Ok expert dungeon crawlers: what do you do when you can't find who/what you are looking for?

The mage won't look any different, but you'll definitely get a message when you kill the right one. When I can't seem to find my target and I'm sure I searched every corner of the dungeon, I search another hour or so. If it's still not enough, I exit the dungeon and re-enter, starting the search anew. If worst comes to worst just let the quest expire, it's no big deal in that game.

I've found 3 reasons why I couldn't find a quest object. (Assuming I'm in the right dungeon).

1. Missed a room/section
2. A glitched dungeon module. (I've found 2 different dungeon modules that, at times, have sections completely separated from the rest of the dungeon).
3. Quest object falling into the void from a high place or down a shaft.

I just visited the Lysausa Coven in Wayrest. Neither 2 or 3 above are applicable. I suspect you just missed a room. Assuming you have CHEATMODE 1 entered in your Z.cfg file you can find the mage by using [ or ].

I know of only 1 quest where you do not get a pop up window telling you that you've found/killed the quest object. It's a Fighters Guild quest where you're after a Lich.

Have you seen any teleporting walls? Those dungeons get confusing. Have you seen any wooden trapdoors on the floor and need to look for the switch? If you get too desperate, voidwalk to find them. Being in the habit of whipping out the map to check for hidden doors helps too. If you found one of those rooms with eight switches, you'll have to find the right combination to proceed.

And just to be sure, do you know all the types of dungeon teleporting systems?