The Ark:

1. Start the mission and once you enter the first building take a left into another door and now you should be in a curved corridor. On the inside is a Terminal.
2. After activating the bridge to let your comrades accross the gap, do a 180 and you should see it. (it does not open until you activate the bridge)
3. In the third building after defeating the scarab, kill the group of sleeping covenant, then follow the corridor downwards. Once you reach a door in front that is locked, immediately on the left there's an open door. Go through and walk straight off, then do a 180 to find a secret room. It is in there.

The Covenant:

1. When in the first tower standing on the lift, face the access panel and turn left. Jump over and it's right there.
2. Land your hornet on the second tower, walk towards the entrance but when you see the locked door, do a 180. Should be right in front of you.
3. When in the third tower standing on the lift, face the access panel and turn right. Jump over and it's right there.

Halo:

1. After reaching the end of the first cave, hug the right wall and you will see a building. Jump up onto the walkway and hang a left once inside. It's right there.