Saighead

Just to add more to the pot: I arrived in the area without the quest in hand, and cleared out the lizard camp just south of the cave's entrance. Don't ever recall seeing a shade to follow into the cave then, but I didn't enter the cave at that time regardless. Came back later with the bounty quest in hand, entered the cave (still never saw or knew of a shade to follow in), nothing ever spawned within. Stuck now, for my character, it's just an empty cave with a quest area marker, and nothing happening.

As for how you received it, it was probably when you accepted 6 quests from the bounty board in Fior mes Iverno all in one go. Five of those were bounties, the 6th is this map's breadcrumb for its missing cartographer.