Halo 3: ODST takes place directly after the Prophet of Regret bugs out of New Mombasa, it's not too surprising to learn that Master Chief won't be appearing in the game. See, everyone knows that the UNSC In Amber Clad -- carrying Master Chief -- jumped into the same slipspace rupture used by Regret. Duh.
Still, if you were sternly hoping that John 117 might make some kind of cameo, those hopes have now been officially quashed by Bungie's Brian Jarrard. Speaking to Multiplayer, Jarrard stated the totally obvious, "Master Chief, at the time this game takes place, is actually off on other parts of the galaxy." Jarrard elaborated, "In this game, it's all about you playing the role of an ODST," adding, "The way the timelines work in this game, Master Chief is off doing something else."
So there you have it, the only place to find Master Chief while playing ODST is where he can always be found: Our hearts.