Here is an interesting comparison of Cortana over the course of the 4 halo games:
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Hidden Picture of Beaver Creek in Battle Canyon
Level: Battle Canyon
Hidden in Battle Canyon, there is a hidden picture of Halo 2's Beaver Creek. It is a small picture, but it is on the underside of the structure hanging above Red Base. There are more of these hidden pictures, surprisingly enough, a nice memorial to Halo 2, but weird because this game takes place before Halo 2.
Good hunting! :)