This one is in Halo 2 on the multiplayer Containment (Level), where a face can be seen in the Halo ring. The face resembles Frankie of Bungie Studios.
It can be found by standing near the red team's base where the turret is, then zooming in with a sniper rifle at the ring (Even though the Sniper Rifle Scope isn't required, it may be seen through regular zoom).
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