Hidden skulls were firs introduced in Halo 2 and ODST once again brings with it a unique set of hidden skulls to find. There are three total, each hidden inside one of the new multiplayer maps. Fortunately, you don't have to search for the skulls while in the midst of a multiplayer battle—in fact, you'll need to find them in Forge mode (Forge is available on the multiplayer disc). Load up the level and use the flying camera mode to navigate to the skull locations. Once you've found the skull, position yourself above it, toggle out of the flying camera mode, and hold whichever button you've got mapped to picking up weapons (default is RB). Your character should fall past the skull and pick it up on the way down—if you miss it, just try again.