You will unlock a secret achievement by finding all seven terminals listed below...

Terminals: The Ark
Go left through a door once in the very first building of the stage. The terminal is located beyond a curving passage. A terminal is located right near the device that activates the bridge on this stage.

There's a secret room in the third building which holds the third and final terminal on The Ark. After you come across and kill a group of sleeping Covenant enemies, head down the nearby corridor. You'll reach a door that is locked if you do. To the left of the locked door is an open door. Head through the door, and then turn around to find the terminal.

Terminals: The Covenant
Head to the stage's first tower and stand on the elevator. Face the control panel, and then turn left. Jump over the obstacle, and find the terminal you seek.

Head to the stage's second tower and walk towards the entrance. When you come across a locked door, turn around to find the second of three terminals here.

Head to the stage's third tower and stand on the elevator. Face the control panel, and then turn right. Jump over the obstacle, and find the third and final terminal on the stage.

Terminals: Halo
Traverse the first cave, and then head right until you come across a building. Walk along the passageway towards the building. Once inside, run left to find the terminal.