That claimable base in the slums, which was basically just a random abandoned house. Keys to the place kept getting passed down from faction to faction to faction, I don't think it ever was without an owner a single time.
But I was told that if ever it was without an owner, there was an npc you could get the key to the place from.
Who was that npc? I never ever found out who it was, and assumed that not being able to find any npc with the dialog option to get a key, meant that the place was always owned by someone.