I had a random dreg on patrol poop out an exotic engram. I kid you not. 2,549 hours into the game. Did not even know this was possible...
Link is to a video of this occurring I transferred from my Xbox One directly to my YT channel with no editing whatsoever. Original is still in the Xbox cloud storage so feel free to view there as well.
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Three of coins increases the chance, meaning that there must be like a .000000000001% chance without it. You got very lucky for it to drop from a dreg, not an ultra. Nice job