What are unmarked quests?
Skyrim contains a number of unmarked quests and locations. These are hidden and will only reveal themselves upon discovery of a certain area or after collecting a certain amount of items.
More here:
https://screenrant.com/skyrim-hidden-quests-experts-find-tips-hins/
and here:
https://elderscrolls.fandom.com/wiki/Unmarked_Locations_(Skyrim)