I saw someone in the original thread saying his activity history suggested The Coming War as the most likely place if he's lying about an engram drop(which seems likely considering it's the only, I repeat, only reported case of a hidden exotic dropping from an engram). The armor resembles cabal gear, the perk is like their jump jets, the name "Garrison" is generally used with Cabal stuff above others, and the word "Twilight" could mean plenty of things other than referencing the Gap; for example darkness falling in the first TTK mission. That's the theory I'm buying, personally. When we see a second guy engram it and show video of decrypting it the original owner will be proven innocent, until then he just doesn't have any real proof given.

I got alchemist raiment from draksis