Sealed DM has 3 possible instances, accessed at random: defend against waves of mobs, hunt the experiments, and gather the keys.

I have never once obtained a hidden prison key for the final door. People have said the final door is the one that gives manastones, and coins/BMs. I'm happy with the AP from the other 7, but I feel i'm missing something when people talk about getting the hidden key frequently when I've run at least 10 key hunts and never got one.

I've noticed there are 2 kinds of boxes in the instance. when you hover your mouse over them, one shows up as Ancient Box, and the other shows up as BOX. After my first few runs I noticed I wasn't getting anything from the BOXes. I figured BOX was a trap to waste your time and decided to avoid them and only loot Ancient Boxes, but I'm starting to wonder if perhaps the Hidden Prison Key only drops from BOX and i've been gimping myself this whole time? or am I just unlucky?

I can typically keep 3 crystals alive in wave version, and I can kill my way to the prison in excapee hunting, which is enough to get the reward box, but I just can't find the hidden key and it is gimping 1/3 of my runs each week. Anyone have any tips?