I recently started playing and just completed the first dungeon, Iron Outpost. Actually, I'd say I've discovered everything (and the game also says 100%), but there are still three white exclamation marks on the mini-map (game instructions p. 6: "dungeon event") and two "magic shrines". And according to the book I'm still missing one of the 12 sagas.

What is a dungeon event?

In the dungeon, I find "fountains/diving basins" that look a bit like the shrine icon, but I can't interact with them. Do I need to unlock anything first?

I found two secret walls. But since Gully can only use Ground Smash five times, I don't want to try finding more secret walls randomly: how can you identify such walls (except fog)?