Floodgate (or The Storm)
At the very beginning of "Floodgate" you'll walk forward, up a small ramp and you'll see a fence with a destroyed ship behind it. Talk a left turn towards the next checkpoint, and there will be a narrow pathway with small indents on the side. In the first "hole in the wall" on the right hand side you'll see a Missing Person
poster on the ground of Jason Jones. Look at it and the achievement will unlock.
This can also be found in The Storm
in the exact same spot, near the end of the level. As you approach the hologram of the Prophet, you'll take a left turn and end up in the same area as above. Because you approach from the opposite side, the poster is found in the last "hole in the wall" on the left side.Note:
Not possible in split screen!