Now you see me, now you don't
Fancy going invisible for a while, to sneak past enemies or do some mischief? What you need is a Covenant 'active camouflage' device; a glowing blue ball in a transparent pyramid, which you'll find in a few select places. Step over one and you fade out over about 5 seconds. Full invisibility lasts for about 50 seconds, after which you fade back in again; so you'd better be in safety by then! It would've been handy if this thing bleeped to give you some warning it's about to wear off; but no, you'll just have to judge it as best you can.
If you're sneaking past enemies while cloaked, don't brush against them else they'll sense you and may open fire, perhaps also provoking a volley from other enemies. If you take enemy fire, you start to become visible again, which could be the death of you; so you need to be careful. Also, leave your flashlight off (not that it's any use when cloaked) or enemies may see you.