It's possible the very last pair of skeletons, to the right of the catwalk on the way out, are the Phantom and his detective. One, the bodies are both clothed and whole, in stark contrast to the artistically arranged skeletons and pieces elsewhere. Two, there is a machete and piece of chalk lying next to them, perhaps suggesting a murder-suicide. Three, the 4th holotape implies the phantom intended to stay with the detective down in these sewers for a long time. And four, the dungeon itself seems very incomplete without some kind of finisher; you traipse through a variety of ghouls to find...nothing except a magazine and some taped laughter? Anticlimactic to be sure, without this theory, anyway.
So if this is a pre-war crime scene, some serial killer's hideout, does Nick make any comments on this when you take him there? Can't try it, I don't have him right now, he is waiting for me in Goodneighbour and i don't want to continue the story quests at the moment