I'm in a similar situation. Here are additional details:
My location is Redoran's Retreat.
I visited this location once, long ago (when I was, perhaps, level 10 or so). I killed all the bandits I found, but I don't recall for sure whether I cleared out all the groceries and such. Now that I've come back (level 30), I find that the mushrooms and groceries have respawned, but not the bandits. There was no opposition in the dungeon at all.
I'm pretty sure that the location marked by the quest marker is the spot where I left the bandit leader's body, on my first visit. It's just as you enter the inner cavern of the dungeon (actually, just inside the connecting corridor).
Clairvoyance agrees with the quest marker: I'm supposed to be finding the Silver Hand Leader right at that spot on the ground.
I have a goodly number of quest lines in some partial state (say, a screen and a half worth of quest names in the list), and also a similarly goodly number of open miscellaneous quests (again, about a screen and a half).
I have visited the Retreat, trying to complete the quest, in company with Lydia, and again with Aranea Ienith; same results.