This achievement is awarded for seeing all flashbacks in the level, The Dead City. The black ghosts do not have to be seen, only the coloured flashbacks. They have to be completed in one playthrough, but you can use level select to do this. The achievement/trophy should unlock after seeing the last flashback in the subway, and hearing the foghorn-like noise. Below is a video guide if you are having trouble finding them all.
Only three flashbacks are actually optional: The piano, the shower and the child. The rest are seen as you progress through the level. Each of these is marked with a specific time on the video to the right.