How can I get all places marked with question marks on the map?

There are lots of interesting places to find, with nice loot sometimes. I've been hunting down question marks on the map, but I noticed that I don't have all of the places marked on my map yet for some reason.

I just destroyed a monster nest that was marked with a question mark, and found another one close by that wasn't marked on my map.

I know that reading the notice board can make places on the map, and I've read them all. But what can I do apart from that to mark all the interesting places on the map?

Solution 1:

So far my map got marked with question marks doing these actions:

• Reading notice board
• Overhearing conversation
• Talk to certain NPC
• Read notice/book you loot
• and obviously quests