This Side Mission begins just south of the entrance to Wonder Tower in the Bowery. While you can find these bodies throughout the game, which are unmarked by the way, the game won't allow you to pursue them until you have a specific number of Riddles done.
You'll come across a dead body, scan it and identify some evidence to follow a trail. After getting to the end of the trail and interrogating the person the Side Mission formally opens.
The next victim is located just to the north of where it says "ACME Chemicals Building," it's a small patch of darkness on the map between two buildings. After identifying the body and materials, you will be done with the second part of the quest.
The third victim is located just south of the "ACME Chemicals Building" on a street. You to climb a fence in order to get to it from the streets. After identifying what's necessary, you'll follow a path to a person of interest who will tell you the location of the killer. After going there, hit on the answering machine to play the cutscene. Afterwards, use your Cryptographer to open the gate and get the Trophy.
Thanks to Xx Overkill VR over at x360a.org for this photo