This mystery can be found in central Amienois, just east of the Compendium monastery in Francia, Siege of Paris DLC.
How to Complete Hidden Justice
As you approach the mystery area, you’ll notice a group of civilians fighting some soldiers. You’ll need to help the civilians and defeat the soldiers, then speak with one of the civilians to learn a bit more about the situation. It turns out that the soldiers are taking too much silver in taxes, and the civilians fought back for it. So the tale goes, or these could be common thieves that you’re dealing with. Whatever the case, in exchange for your help the civilian offers you a map, but states that you have no right to any of the silver. You can choose to take both map and silver if you want, but we asked only for the map, which leads to some hidden treasure.
Where to Find the Treasure at Murmiliacum
Once you’ve completed the mystery, you’ll have the treasure map in your inventory, which reveals that the treasure is located in Murmiliacum, a small settlement to the north. When you arrive, you’ll need to look for the two small buildings in the southern part of the settlement, where a coin mint can be found. You can loot a lot of silver lying around here, but there’s also a hidden chest in the southwestern corner of one of the houses. Looting this provides 1500 silver.