I've completed the game 100%. Some quests were completed with hints from the bartender and fortune teller. Messages from quest-giving NPCs were also taken into account. So, it's not entirely clear why some players are having issues: the bartender and fortune teller have hints for all quests.
It's right there in the name: hidden quest. It means that you have to go out and find them. Just keep interacting with everything and everybody you can and eventually you'll get them. (Yes, it can be frustrating at times. Join the club.)
This forum is also a good place to look for hints. Scroll down and read what others have had to say about it.