There are a total of collectible "messages in a bottle" located throughout
The Hideaway DLC. They are also referred to as "
flotsam" in the game. You cannot chapter select within the DLC, so you must restart the entire DLC if you miss one. Upon collecting all 5, you will unlock the achievement.
1 - Upon reaching the tabletop to free the nome from the cage, work your way to the left and through the hole in the wall to find this collectible.
2 - In the furnace area, you'll want to go up the ladder on the left. Go up the 2nd ladder as well, and find the collectible underneath the staircase nearby.
3 - If you decided to do the left area first (from the above mentioned ladder), you'll work towards opening doors and pushing a minecraft. Push the minecraft all the way to the left, and use it to get jumping access to the shelves to find the floatsam.
4 - After returning to the furnace area and then continuing to the right, you'll work past the janitor into an open area with some large wooden steps at the back of the room. Go under these steps to find the collectible.
5 - Further on, you'll come to an area where you have a trolley that needs to be used to access a nome. Before doing that, climb the safety deposit boxes, go up and to the left, and grab the final floatsam.
Also make sure to grab the
Ashes in The Maw achievement during your playthrough which is missable.