In a game packed full of collectibles, there are five hidden ones that don't show up anywhere on your "View Totals" screen. These are the Pirate Treasures. There are five Pirate Treasures hidden within Yooka-Laylee, one for each of the Grand Tome Worlds. You will unlock three achievements if you collect them all.
For collecting your first, you will unlock the achievement "Private Pillage"
For collecting any three, you will unlock this achievement "Secret Salvage"
And for collecting all five of them, you will unlock the final achievement "Captain's Cache"
For those of you who prefer video guides, Destructoid have created a very easy to follow, and quick guide to finding all five of these.
The first one you'll have the chance to find is located in the very first Grand Tome World, Tribalstack Tropics.
You will need to have expanded the world in order for this to be attainable. Head to the back side of the boss, Rampo's, tower. If you notice, you'll see a little hole with bricks missing, and on the inside you will see a silver skull. Pick this up to unlock your first achievement.
The next one you will find, is located on the second Grand Tome World, Glitterglaze Glacier, I believe you will once again need to have the world expanded before being able to attain this treasure. Head over to Trowzer's location, jump in the water, and dive under and swim to the right. You will see a cave you can now enter. As you make your way through this cave, Gloomy Gem Grotto, you'll eventually find a big orange monster. Near his platform you will see the next pirate treasure up even higher upon a rock.
The third one you will find is located on the third Grand Tome World, Moodymaze Marsh. It is conveniently at the beginning of the level and can be found after swimming through the second tunnel. Once you emerge from the tunnel, look to your right and you’ll spot a building. Head to the building and climb it, or use Flappy Flight. Near the top, you’ll find the Pirate Treasure inside the left smokestack.
The fourth one you will find is located on the fourth Grand Tome World, Capital Cashino. This actually another one available at the beginning of the level. After entering Capital Cashino, spin around and look up above you. There, you’ll see the Pirate Treasure skull floating above a lamp. To grab it, you can either use Dr. Puzz’s Mollycool transformation or Flappy Flight to collect it.
The fifth and final one you will find is located in the fifth Grand Tome World, Galleon Galaxy. You will need to first expand the level before being able to obtain this one. After doing this, head on over to the pink moon and go inside the Velocity Terminal. Rather than using the slide, however, use your Flappy Flight move to propel yourself into the sky. Upon doing so, you’ll spot the Pirate Treasure lurking above one of the small globes at your right. If you've been following along, you will now have unlocked your 3rd and final of the pirate treasure achievements! Congratulations!