Walking in the Dawn
From Halopedia, the Halo wiki
At the very end of the level, at the jump leading to the Forward Unto Dawn's hangar, drive the Warthog at a slight angle at the ramp. As you drive off of the ramp turn very slightly to the right so you will come off a bit sideways. If done correctly you will land inside the hangar without activating the cutscene. Exit the Warthog and you can freely walk around the hangar. It will be empty, save for four pillars and a red light in the middle. Walking further inside the hangar will cause you to fall through the floor to your death.
Note: you may not be fast enough to land in the hangar; instead you might get stuck inside the hangar's wall.
The cutscene doesn't start because the angle you come off of the ramp makes you fly under the barrier that activates the cutscene.