Reunited achievement (secret achievement) 400 days

Very easy to miss this achievement, if you don't know about it.

At the very beginning of Russell's chapter you are walking down a road. When a car comes you are given two options; Stand your ground or Hide.

Choose whichever is your Hide option to dive into the bushes.

Russell then dives in the bushes, and laying there next to you in the grass is the remains of someone you killed off in the game previously. Few seconds later, Reunited pops.

If you miss this achievement, you can easily go back to get it.

Either start a new game, and start off on Russell's chapter, or Load previous game, or rewind(Rewinding will undo all your choices made up to that point), and play Russell's chapter over again.