I completed the mission and found the chest with thorn, but it is not in my weapons nor did it go to Kadi. I can’t go back to the chest so I am at a loss.
Thorn does not come from a chest, you get it at the end of a fairly lengthy quest. I see your most recent game history instance was exploring the EDZ. Did you go up to the abandoned campsite, and search through there? If so, that's just the first step in the quest. Next, you need to talk to Banshee in the tower.