Cortana in High Ground is an Easter egg in the High Ground in which you will be able to see a picture of Cortana for a split second.
How to Find Cortana[]
Starting from the AA missile (opposite end of the beach), go straight. At your left side, there should be an old, half-destroyed bunker with a hole in the side. Go through the hole. Turn right, and a computer should be laying on a desk. Go to the computer, look at the computer screen that shows a picture of the hangar from the mission Crow's Nest.
Look at this screen for a while. Eventually, an image of Cortana should appear on the screen for a split second, and then disappear. The image reappears on the screen roughly every two minutes.
Cortana can be found on any small computer spawned into High Ground via Forge. She can also be found appearing on any small computer in the starting area of the campaign level, Crow's Nest.