So I found these 4 black alien skull looking things (possibly elite skulls) scattered around the area where you have can pilot a banshee to blow up vents. They shoot out white sparks/fireworks when you walk up right on them. The fourth I didn't find until after I had blown up all the vents and it didn't spark. So I am guessing you must do it before then. Might try tomorrow unless someone wants to now, maybe there are more than 4 too.

1. Large curving platform below exit area, right in the middle 2,3. Opposite side from previous if you follow the outside pipes to their terminus and then follow a trail to the outer limit of the walkway.
2. To the right of the exit area on a small Square platform.