https://www.youtube.com/watch?v=faa1XGMocW0 Complete the final mission of the Missing Persons Investigation. There are 6 missing people to find in the Missing Persons Investigation, the locations can be revealed on the map after you unlock every ctOS tower. You then need to go to these locations and scan the dead bodies. Once you have found all of the missing people you will receive a call from the killer and then need to complete the final mission to catch the killer and unlock the achievement. You won't be able to find the final 2 spots until you complete the mission to find Raymond in the 3rd act it will reveal the collectibles locations in the northwest area of the map. Doing this is unecessary as the items still show up and you can get them but its recommended as some collectibles don't show up til late in the 2nd act and activating the tower in act 3 reveals their locations making it easier.