On the level Pillar of Autumn, it is right after you come out of the canyon, and fight your way through on the west side of the frigate that is under construction.
Head up the left side of the frigate, and then go east by the red generator things. By the left opening there is a spartan laser, and by the right opening there is a sniper rifle. Go to the right side, pick up the sniper, and look right above the shade turret. There should be a yellow elite there. If you shoot it with the sniper rifle, it's going to take five times to kill it (haven't been able to yet, though I haven't tried real hard). The only problem is that it disappears after a few seconds.
Has anyone ever seen this before, and what happens if you kill it?