I was doing some patrol missions on Mars the other day. I noticed that on three or four different occasions when I killed a Cabal Phalanx and or a Cabal Legionary, after their body hit the ground, an engram popped out and landed where they were laying. Those engrams disappeared to another dimension of some sort. I lost two blue engrams and a green or two as well. I hope that this will be looked in to at some point, I would really like to go back to Mars one day.
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If the actual engrams dropped and you did not pick them up then the rare engrams will be waiting for you at the Postmaster. The uncommon green engram will not. Although, you should be able to walk through the cabal and pick up the engrams either way. More than likely ammo drops or other effects made it appear as if an engram had dropped.