I've noticed this for the first time today. In Driftwood I found a location which makes it pretty clear there's a hidden treasure there, so I used my shovel on the spot despite my characters being unable to spot it themselves (low Wits I assume).
That's not the problem. The problem is that, although the uncovered terrain features are just grass and flowers, the minimap shows a permanent brown mound even before I dig the treasure out, letting you know there's something there!
I then started looking on the minimap for more of these circular brown features, and voila, whenever I walk into one either you "spot something" and a hidden treasure comes into view, or even if my wits are too low I can still uncover the treasure by digging on the spot.
So basically Wits are useless for spotting treasures outdoors, because the minimap tells you where to find them.
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This is different from going online searching for the hidden treasures. I consider that cheating and so I don't do it. However, I can't prevent myself from not spotting the brown mounds on the minimap. They are pretty obvious. Once you see it you can't unseen.
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The solution is obvious. Since it seems the minimap is static and does not update (or does it?), for the Definitive Edition the minimap should show the terrain features as they are without hinting at the presence of treasures.
If this is confusing I can provide screenshots.
