Active Camouflage 

An piece of covenant technology that renders users of it partially invisable. The UNSC captured a piece of active camouflage, and made their Spartan-II MJOLNIR armor able to integrate itself with the covenant technology. In the Halo game series, players can become invisable if they find active camouflage.

No wonder he was able to get behind our base... he had active camouflage.

Active Camouflage by Drüd October 19, 2008