Hijack
From Halopedia, the Halo wiki
The Carjacking (or Highjacker in Halo 3) Medal is awarded for stealing an occupied vehicle from an opponent. It is available in all Multiplayer gametypes where a vehicle is available and has a Bungie difficulty rating of 2 (out of 10). It is represented by a cross with a U-turn sign on it. After you steal an occupied vehicle, a message appears on your screen that says: "You boarded an enemy vehicle".
