A skull similar to the one that activates blind mode can be found if you Start on "regret". After the first lift ride (before the first underwater elevator), grenade jump from the front awning of the lift to the ledge in front of you (the one with two turrets).

Grenade jump AGAIN onto the next ledge.

Make your way to the back of the building. On Easy, you'll only find the skull. On Legendary, however, 2 invisible Elites carrying swords just shake their heads wildly, ignoring you completely.