When completing this quest, 2 of the 4 black marks apparently fell under the chest instead of going into my quest when looting. If it helps, I believe I had a full inventory when it happened, even though the black marks don't go to your inventory- I think there is some accidental inventory lookup going on for the black marks. I can physically see the shining underneath of 2 of the chests, with 2 on my character. So I know they are there, but I can't get to them because the chest is in the way.
I ended up leaving the game, coming back to the spot, and the chests were sealed again. I still randomly had 2 keys and opened them, and they were empty.
I left again and came back, now I have no keys and those two are locked again. Picture is what they look like (the shimering as if there is something underneath it)