This is a great reason for having a lock picking skill. I was in Cadmus' cave and encountered a group plunder chest. Since this is a solo cave there is no way I can get any help to open it. Here is a shot of the paradoxical situation.
I also found a juicy four lock chest in the Archery Range. It was SO hard to just sigh and walk away from such riches.
Unless this is a bug...............It sure made me want to pick those locks, lol.