This glitch only works on Halo: Combat Evolved and Halo PC.
To see a Stunned Marine, you must play The Silent Cartographer on any difficulty. There is a glitch that makes marines freeze, and you will be able go through them.
First take a Warthog and fill it with Marines. Then, take the Warthog to the door you are supposed to open, and leave. Then open the door. Once that's done, go to the door again, and ask the marines to step out. If you've done it right so far, they should still sit there. Go out and get a new Warthog and drive back there. Now the Marines should be out. Hit them with the new Warthog, and their weapons will fly out of them! They will just stand there and be like ghosts. But if you activate the Silent Cartographer they will disappear.
It is probably due to how the Halo: Combat Evolved engine handles loadpoints and the trigger areas of loadpoints, where you, the player, travel to another loadpoint (the Security center), but the Marines do not follow. When you left the loadpoint trigger area of the Silent Cartographer and went into the Security center area, the AI got removed from the Marines in the previous trigger area to save memory. That is the probable reason why this glitch happens.