Hah! Finally found it!! I started going through all the rooms below the city where I hadn't found any loot before. Figured that would be the most likely way to come across it. Man that was a tough one.
I also found the last nugget in mission 2. It wasn't in the water at all, so I had been searching in the wrong place all along. The loot list didn't give anything away there; it was more misleading I'd say.
Then I have what I need to start my ghost runs. Wish me luck...![]()