The other day in fel, I saw some folks dig up a chest. Naturally, I chased them away and called a friend who had lockpick and remove trap. As it turns out, you cannot steal people's treasure chests like that. I think it let us remove the trap but wouldn't allow us to pick the lock. I got bored waiting for him to return, it was a low level map anyway. The plan was to wait for him to return and kill him just for fun, but I don't know that he ever came back.
There was another time I came across a treasure hunter that had already unlocked the chest. I killed that one, and I was able to loot the remaining treasure from the chest.
So your treasure is safe until you unlock it.