These vents are where you see the first Zealot after the cutscene. The one in that long hallway that has the gas tanks on the right side, and it's got a concussion rifle. Instead of taking the right-hand turn to follow the Zealot, keep going straight. You can stack boxes to get near a ledge up high, but you need to stand on someone and do a double-jump in order to make it up there. Then there's windows at the ledge, made of unbreakable glass. You can't get in there on your own, but if you kill the person you stood on to do the double-jump (ie., ME!), they'll spawn behind the glass in an empty room. It's kinda small, and completely empty, but maybe we can find something, somewhere, if there are these rooms in other locations as well.
I'll try and get screenshots and a video of it, but it's at the very end of an hour-long video, so we may need to get a shorter video made. I don't want to do a half hour's worth of fast-forwarding.