First you need to see items through walls (Level two of the eye)

During the first few times in between missions head out to the street and go look at the giant barrier on the left. Turn the power on and there appears to be either bolts, or key behind this gate. Now I've come across a few items that are NOT possible to get, because their placement is off in the world that they are actually IN the walls. Curious if this is one of them.

Thought about trying to utilize the very high balcony up to the left to get to it, but even with double jump and Blink 2, I can't get up to the balcony. Also, there is probably a level "invisi-wall" above the gate.