I had a theory i wanted to share,maybe the BELIVE grafitis in mombasa streets lead to the sword or a clue to it,the first thing that made me think this was the fact that the only swords seen in the game are in front of the first BELIEVE grafiti on the T.plaza Buck flashback then i found something really interesting on halopedia a while ago:
http://halo.wikia.com/wiki/Believetwords the bottom of the page it states in Fact that there are SEVERAL Believe grafitis in the mombasa streets map and i only know of 2 that have been found since the games been out,the one by all the dead elites and the one that has 117 next to it in bulletholes i thought it would possibly be worth looking into and maybe doing some searching for more of the BELIEVE grafitis when you go to the one that has 117 that sparkly stuff thats in D04 starts floating around also...ny way thanks and good hunting!
(oh and this is what it says about it exactly on halopedia)
At the Halo 3: ODST map Mombasa Streets, there are several graffiti where you can read BELIEVE, maybe being a reference to the marketing campaign for Halo 3 of the same name.