Saw someone saying on his thread that based on activity one of the likely possibilities was him getting it from The Coming War somehow assuming it really wasn't an engram drop. It does look like cabal armor and the perk resembles their jump jets so that might work, garrison sounds like a word that'd be used in association with Cabal gear, and rather than Twilight Gap it could be referring to something completely different(darkness coming if it's the first mission of TTK, or maybe even just time of day during the mission). If we don't see more start popping up before the mission goes back to daily it may be worth looking into.