It's not Promethean Vision (thankfully). Turns out it's part of a new Spectator mode.
Not sure how I feel about this feature though. What's to stop players from being in a party and abusing it? ""Hey, Jason! Be careful! I can see him crouched at the end of the hall!"

: Not much different than sitting next to the overshield in previous games and
: picking it up when an enemy approaches in order to utilize the brief
: invincibility you get while it charges. The only difference is that you
: can carry the OS around now (and it looks like you can't shoot until you
: activate it).

Turns out it's not a carry-on shield. It now works like the Overshields in Halo4. You press the switch to pick it up (which turns it on right then). But because we've never had the switch animation before, it threw me off. 343I needs to lay off on the switch animations. They are pointless and take you out of the game. And while you're pressing that switch, you can't fire, so what's to stop someone from killing you while that animation is playing?