For this achievement you will want to load up Floodgate on Halo 3. This can be done on any difficulty.

Start the mission and continue as normal, taking a left right at the crashed ship.

Along the sides there will be units, some of them will be open. You will want to enter the first one on the right, and on the floor you will see a Missing Person poster.

Look at in for a few seconds and the achievement will unlock!

Helpful: This can also be done on the mission The Storm, in the exact same spot near the end of the mission. When you go to the hologram of the Prophet, take a left and you will enter the same area as above but from the opposite side.

Fun fact: The person on the poster is Jason Jones, one of the founders of bungie!