Hidden treasures are buried caches across Vale Sangora, usually found by following a treasure map. They are worth chasing for strong gear, including at least one legendary greatsword.
Loot every chest at bandit camps, since that is where most maps turn up. Each map shows a landmark such as a bridge or a swamp edge near the X.
Travel to the sketched spot, match the landmarks, clear any enemies and dig with a shovel. Bring one with you so you do not have to search for it on site.
Caches can hold valuable gear. One guide reports a legendary greatsword in the Briar Sloughs treasure, for example.