From Halopedia, the Halo wiki
The password-lacking Marine is an Easter egg in Halo 3 campaign. In the level Crow's Nest, there is a Marine at the end of the first long, curved corridor. There is a humorous exchange of words between this Marine, who needs ammunition for himself and his comrades, and someone behind a door that leads to an armory. The exact discussion changes depending on difficulty, but it always involves this Marine trying to get through the door without a password that is intended to keep Brutes from entering. Either he forgot or never knew about it. They're voiced and possibly written by the creators and voices of Red vs Blue. At the end of each conversation, the Marine repeatedly knocks the door until killed.
Interestingly, the man behind the door does not even bother to check the security camera even though a sign next to the door clearly indicates there is one, although based on the complaints regarding Crow's Nest frequent maintenance needs, it is possible the camera is offline and the man behind the door used this as an alternative. Ironically, if the player kills him he drops an Assault Rifle which is contradicting as he says he needs ammunition and it actually had ammunition. Also, he won't groan or flinch if the player shoots or melees him.