I've encountered a chest that I've spent the last three days searching for the key for using both the ingame clue and 'super vision' - but the key is apparently non-existent (or the clue is outright wrong).
The chest in question is the one due west of Readigum Abby, at the farm house surrounded by bandits. The letter inside the house states, rather explicitly: "I've found solace near the southern great wall of Winchester"
Except, searching along the southern wall of the city, both inside and out, yield nothing. There are no keys in the houses immediately outside the southern wall. All the keys within the southern wall are for obvious locations like the warehouse, etc. On a whim, I did a full search of Witan Hall, the Old Minster, the Seminary, and the Bishop's residence, also turning up zero. I then searched around the rest of the city walls (West, North, and East), both inside and out, and have found nothing.
Either the key does not exist, is not where it is supposed to be (per the clue), or its tied to a quest (at which point, why have the clue)...