Hidden Quest are quest that doesnt show up in your Find Quest list and are given by an npc you dont get to meet in your route while leveling. They hold great rewards by usually just go and talk with npcs. However these will only triggers and should be completed in one level.

Here are the 2 i found so far, i dont remember if there is another one. See ya.

LEVEL 50
"Hidden Quest": Scarlet Gate

1. Speak to the Elder of the Streams (130, 858) Thousand Stream City
2. Go to the Scarlet Altar (104, 513) Quest will auto-trigger the continuation when you get close to the actual stone. Must be standing STILL in order to get the quest continuation
3. Report to the Elder of the Streams (130, 858) Thousand Stream City
-Collect 5 Bloody Jades (Farmable material. Located around [104, 513])
4. Report to the Elder of the Streams (130, 858) Thousand Stream City
5. Speak to the Etherblade Arena Manager (450, 879) Etherblade
6. Speak to the City of the Lost Arena Manager (245, 639) City of the Lost
7. Speak to the City of the Plume Arena Manager (336, 432) City of the Plume
8. Speak to the Archosaur Arena Manager (531, 630) South Archosaur
9. Report to the Elder of the Streams (130, 858) Thousand Stream City
-Reward (Gold: 85200 Exp: 142000 SP: 32370)

LEVEL 60
"Hidden Quest": Weird One

Note: You don't have to talk to Fengshuist Zhu at all if you want. You can just go collect all the items and then go talk to Stranger Icecold

1. Speak to Fengshuist Zhu (138, 671)
If you pay 5000 coins to him, he will tell you the locations of the items. You can still obtain the items without paying him for the information.

2. Collect 3 items
-Auto-pop up at [113, 685] On the ground right by the pillar
-Obtain Devilbinding Rope
-Investigate the Box from Town of Arrivals (119, 639) Town of Arrivals Basically up on top of the Town of Arrivals. Elevation 63.
-Obtain Devilslaying Bell
-Auto-pop up at around [171, 623] Elevation 62 [color=red]On the ground. Basically the same as the location for the Token of the Seven, Blade of Seal quest. (Quest "Blade: The Seven")
-Obtain Devilconquering Umbrella

3. Report to Stranger Icecold (135, 669) Town of Arrivals
-Reward (Coin: 6500 Exp: 35% of the current Crazy Stone's Exp amount SP: 1/5 of the Exp recieved from quest Rep: 5)