Security helmet? I know that the shoulders are bugged, but the helmet? It's prob because you glitched it, or whatever. If you got it by getting 1,000G in Halo 3, then it should show up. The shoudlers can sometimes disappear after you've unlocked them, or not even unlock at all. They are supposed to unlock at 750G, but it's been reported that they can appear at any time between 400G-1,000G and then disappear right after you unlock them. They'll usually reappear after you get your next achievement. Try that with the helmet; get an achievement on H3 and see if it comes back.