Even if they become available early in the game, the Watch Dogs Missing Persons Investigations can be completed only after Aiden meets Kenney during the third act of the game.
The first Missing Persons Investigations in Watch Dogs appear on your map during the first act, but in order to get the Darkness Looms achievement/trophy, you have to finish the final Missing Persons Investigation, which is triggered after you complete Hope Is A Sad Thing main-mission in Act III.
Similar to the Burner Phones Investigations, the Missing Persons Investigations require your presence in different districts, where your goal is to locate and scan the dead bodies found in specific areas.
Once scanned, Aiden will get one step closer to the criminal, and eventually, you will have to find him. The video game includes 7 Missing Persons Investigations; however, on your map, you will only see 6, all of them revealed by the maps below.
The seventh investigation is in fact the final mission, unlocked after you finish the previous six investigations and Hope Is A Sad Thing main mission. At that point Aiden will receive a phone call from a stranger and a new icon will appear on your map, in Pawnee District.
An important fact you have to be aware of is that along with other collectibles (except for City Hotspots) the Missing Persons Investigations in Pawnee District wont appear on your map, until you unlock the ctOS Tower while trying to find Kenney.
Nevertheless, you can still complete them earlier, if you follow the maps below.
When working on finishing a Missing Persons Investigation in Watch Dogs, you have to look for the corpse of a victim in one of the spots on the maps below.
The districts you have to visit to play the Missing Persons Investigations are: Parker Square (1 Missing Persons Investigation), The Loop (1 Missing Persons Investigation), The Loop (1 Missing Person Investigation), Brandon Docks (2 Missing Persons Investigations) and Pawnee (2 Missing Persons Investigations).
Missing Persons Locations Guide
Watch Dogs Missing Persons Investigation 1 * Parker Square District
In Parker Square District the Missing Person Investigation is located west from the second ctOS Tower, as you can see on the map.
The location is marked by a magnifying glass. If you travel to that location you will find some train tracks and a tunnel. Enter the tunnel and move north around the trains to find a crate.
On it is an Audio File: Unknown 01 and behind the same crate is the first missing person, on the ground.
Watch Dogs Missing Persons Investigation 2 - Brandon Docks District
In Brandon Docks, as you can see on the map below, you have to find not one, but two missing persons. The first one is located in the northern area of the district, near a bridge.
Travel to the location on the map, and then check the area under the said bridge. Vault over the small wall and scan the corpse on the mattress. Next, scan the audio log to receive Audio File: Unknown 02.
Watch Dogs Missing Persons Investigation 3 Brandon Docks District
In the southern area of Brandon Docks District is the third missing person.
When you reach the location on the map, you will find a demolished building. Head inside and try to get to the second floor. You can use the forklift, or you can find another way.
On the second floor, you will find the corpse near a camera. Scan it, then turn around and pick up the audio log on the brown couch (Audio File: Unknown 03).
Watch Dogs Missing Persons Investigation 4 - The Loop District
In The Loop District, the missing person is easy to find, being located on the eastern island, where the lighthouse can be observed. Take a boat, and when you reach the said island, check the small shack, close to the yard of the lighthouse.
On a table you will find the corpse, and in the corner of the same building, on the ground, you will find Audio File: Unknown 04.
Watch Dogs Missing Persons Investigation 5 - Pawnee District
In the northern area of Pawnee District, is the fifth missing person.
In the location marked on the map you will find a train. One of the wagons has open doors. Use the small ramp to get inside, then turn right and you will find the corpse and next to it Audio File: Unknown 05.
Watch Dogs Missing Persons Investigation 6 - Pawnee District
For the sixth missing person in Watch Dogs, you have to travel to the southern area of Pawnee District. Marked on the map is a hunting cabin and the corpse is inside, on the ground, near a chainsaw.
Audio File: Unknown 06 can be observed nearby, on a shelf. Assuming that you have finished Hope Is A Sad Thing, now you should receive a phone-call and the final missing becomes available in Pawnee. Head to the marked location and start the final investigation.
Your goal is to tag the woman nearby and stay close to her. Follow the dirt path down the hill to spot her. Hide behind the fence around the water pool and wait there until you are informed that you should intervene.
Rush towards the killer before he hurts his victim and take him down. If you dont shoot him, you will receive a better score as a vigilante.