You can start this quest after visiting Kadara Port for the first time during the quest “Hunting the Archon”. Return to the Operations deck of the Nexus and head to the bottom floor of the Pathfinder Headquarters to find a Turian named Sellers, who is distraught over the disappearance of their friend, Sanjiv Clement, who she claims was abducted by his fling, Delany Roos.
This quest requires you to travel to Kadara Slums, so despite starting in the Nexus, it’s recorded here. Also, you don’t actually ever need to properly start this quest. Sellers, the Nexus, the whole sob story? Irrelevant. Just head to the second story of the building complex that houses the Tartarus and make your way to a dark alley north of (around the edge of) the bar, where you’ll find a corpse and a Datapad. Scan the former and read the latter to identify the body of Sanjiv Clement and clear his name. This will earn you 530 XP, and if you want to properly end the quest you can return to Sellers in the Nexus.