On the level where you bring down a sheild, the first elevator you reach has a ledge on the side that you can jump on. Hop onto it and go to the opposite wall where a panel will open up, enter it and you will be able to read a Covanants soldiers diary.
On Seirra 117, once the two pelicans are shot down, you progress into a part of the forest where snipers pin you down. Stay to the extreme left and follow the wall and hop up on the rocks in your way. Once on top, look left and you will see a family of crazy monkeys holding a teddy bear.
Unlockable: Gold Skull Locations
Black Eye (Located: Crow’s Nest)
When activated, you will have to battle enemies with melee tactics to regain shield energy.
Catch (Located: The Storm)
When activated, enemies will throw more grenades.
Famine (Located: The Ark)
When activated, dropped weapons will hold less ammunition.
Fog (Located: Floodgate)
When activated, the motion tracker disappears.
Iron (Located: Sierra 117)
When activated, you will be sent back to the beginning of the stage you’re on when you die instead of to the last checkpoint.
Mythic (Located: Halo)
When activated, all enemies will have twice as much health.
Thunderstorm (Located: The Covenant)
When activated, enemies will be of a higher rank.
Tilt (Located: Cortana)
When activated, enemies will become more resilient to your attacks.
Tough Luck (Located: Tsavo Highway)
When activated, enemies are skilled at getting away from projectiles such as grenades.