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i have been finding quite a few caves, catacombs, ruins etc on exploring the whole area which dont then show on the map as discovered. i am assuming this is how the new game will be. i have not found them as part of a quest.
Not all caves and ruins have map markers. Here is a list of all locations in the game (excluding Vvardenfell and Summerset) which do not show up when discovered; this is not new to Summerset.