It reminds me of a series of murders that took place around 5 or 6 years ago. Some guy decided to start bludgeoning homeless people to death in their sleep. Not every homeless person lives down in the tunnels. These victims were in more public spaces which is probably the only reason it was ever eventually investigated.

But the police had no leads. So they set up a dummy in a sleeping bag and waited for somebody to come along and bash it with a hammer. Eventually they got their guy.