Unlocked during campaign mission: Another Life
After you've completed all objectives and are ready to leave, don't use any of the exits on the map. The one we want to trigger for the achievement is hidden.
1) Grab the paddle
. This item is located in the garage of the house for sale. The house is labeled as the Schmidt Property on the map. Use a lockpick from your inventory or the key that is hidden in the right-hand flower pot near the front door.
2) Head towards the southernmost part of the level and run along the creek/stream
. Be sure to keep the paddle equipped otherwise the hidden exit won't appear. Stick to the edge of the creek until you spot the new exit symbol and use it to end the mission. An achievement and the corresponding in-game challenge will unlock.
If you're more of a visual person and would prefer to follow a video guide, here is one courtesy of Maka91