Cyrodiil is vast and full of secrets. Many quests and locations are not immediately obvious and require exploration or keen observation to discover. For example, the 'Whodunit?' quest for the Dark Brotherhood is a unique murder mystery that can be missed if you don't follow the contract's specific instructions. The 'A Brush with Death' quest in Cheydinhal leads to a magical painting and a unique staff.
Hidden locations include numerous unmarked caves, Ayleid ruins, and forgotten shrines that often contain valuable loot, unique enemies, or even mini-questlines. Look for unusual landmarks on your compass, talk to every NPC, and thoroughly explore every dungeon. Some quests, like 'The Ghost Ship of Anvil', are initiated by reading specific books or notes. The 'Deepscorn Hollow' and 'Battlehorn Castle' DLCs also introduce hidden areas and unique player homes with their own mini-quests. Don't be afraid to stray from the beaten path; you never know what you might find!