Every stage from Kai (chapter 1-2) and onward, with the exception of the tutorial battle, Yomotsu Hirasaka and Hana Cave, has 3 treasures other than Wicker Boxes that are hidden and buried in the terrain and searching for all 81 hidden treasures will unlock this trophy. It is recommended to combine this trophy with Silver Rabid Curiosity and Gold Protector of Mizuho.

Hidden Treasures will not be visible on the map, instead a character will need to either use the Search command to unearth it on certain panels or use the item Augury Skill, which is a shop-bought item for 20 leaves each that when used, will give a hint if there is a hidden treasure on the panels you used it on. The third option is the Monk Job's Mine Investigation skill, which does what Augury Skill would do perfectly, but with less panels included. Due to hidden treasures not being visible on the map and would require some work to unearth them from scratch, it's recommended to not bother with them, especially on Boss battles, and instead focus on finding them while doing shrine requests.

Below is a screenshot list of all hidden treasures' pinpoints in each stage plus the Shrines that have the stages replayable in some requests, but do note that Hidden Treasures do not regenerate once unearthed. Luckily, you can make a checklist of how many Hidden Treasures you found using that tip. Click on the images to enlarge them. Credits to user on PSN: StilRh for providing most of the screenshots.

Kai (accessible via Kagamihara Shrine's requests)
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Kiso River (accessible via Kagamihara Shrine's requests. Third image requires the passive skills Water Run or Flight Spell to reach)
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Ibuki Mountain (accessible via Ooe Mountain Shrine's requests. Third image requires the active skills Shadow Skip (Shinobi), Transfer Time (Confucian) or the passive skill Flight Spell)
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Ooe Mountain (accessible via Ooe Mountain Shrine's requests)
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Daisen (Mountain Springs ver.) (accessible via Yoshida Shrine's requests)
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Hii River (accessible via Yoshida Shrine's requests)
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Yoshida (accessible via Yoshida Shrine's requests, unlocks in chapter 3 or 4)
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Itsukushima (accessible via Saga Shrine's requests)
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Dan Bay (accessible via Saga Shrine's requests. All three treasures can only be reached by having the passive skill Flight Spell)
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Saga (accessible via Saga Shrine's requests)
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Kusunoki Village (accessible via Amakusa Shrine's requests)
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Daikanbo (accessible via Amakusa Shrine's requests. Second image has two hidden treasures glowing)
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Mount Aso (accessible via Amakusa Shrine's requests)
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Oosumi (accessible via Ryugu Shrine's requests)
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Shiomitsu Bay (accessible via Ryugu Shrine's requests. Third image may require High Jump)
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Gokase River (accessible via Usa Shrine's requests. Third image requires the passive skill Flight Spell to reach)
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Hyuga (accessible via Usa Shrine request)
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Manai (Waterfalls ver.) (accessible via Usa Shrine's requests. Second image requires High Jump, Third image requires the passive skills Flight Spell to reach)
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Manai (Shrine ver.) (accessible via Usa Shrine's requests)
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Yamato (accessible via Fushimi Shrine's requests)
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Kibi (accessible via Oni Castle Shrine's requests)
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Oni Fort (accessible via Oni Castle Shrine's requests)
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Oni Castle (accessible via Oni Castle Shrine's requests)
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Daisen (Erupted Volcano ver.) (accessible via Yoshida Shrine's requests in chapter 4)
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Kanba (accessible via Ooe Mountain Shrine's requests in chapter 4. Unrelated to hidden treasures but a Rare Wicker Box in this stage can be reached from platforms that are next to shallow water only)
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Tsukuyomi (accessible via Ooe Mountain Shrine's requests in Chapter 4)
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Fuji (accessible via Ise Shrine's requests)
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Ise (accessible via Ise Shrine's requests)
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*Daisen has two variations of the stage, and both of them have 3 different wicker boxes and 3 different hidden treasures each. Daisen (Mountain Springs ver.) is the stage encountered in Chapter 1 Episode 8 while Daisen (Erupted Volcano ver.) is the stage encountered in Chapter 3 Episode 11. Both locations can be found in Shrine Requests as with all 25 other stages previously mentioned after they are cleared in the Story.
**Manai has two variations of the stage, and both of them have 3 different wicker boxes and 3 different hidden treasures each. Manai (Waterfalls ver.) is the stage encountered in Chapter 2 Episode 12 while Manai (Shrine ver.) is the stage encountered in Chapter 2 Episode 13. Both locations can be found in Shrine Requests as with all 25 other stages previously mentioned after they are cleared in the Story.
***Skills and passive skills that are mentioned or helpful for traversing maps:
Shadow Skip (Shinobi): Allows the user to transport to up to 5 spaces further. Shadow skip ignores height difference and bypasses enemies in the way, but cannot enter Medium or Deep level water (unless the user also has Water Run, Under the Sea or Flight Spell passive skills equipped)
Transfer Time (Confucian): Allows the user to transport to up to 5 spaces further. Transfer Time ignores height difference and bypasses enemies in the way, but cannot enter Medium or Deep level water (unless the user also has Water Run, Under the Sea or Flight Spell passive skills equipped)
Water Run (Sea Captain, Sayori only): Allows traversing in shallow and medium level water terrain, plus increases movement in water. Flight Spell is a much better choice just because even with this skill, you still can't enter deep level water.
Under the Sea (Kame, Kame only): Able to enter all three levels of water terrain.
Flight Spell (Tengu): Allows the character to bypass characters and objects in the way and provides the ability to fly above any level of water terrain, plus negates height difference blockage of any weapon when attacking. Height difference still applies though so you can't jump to a higher elevation if you don't have enough Jump power.
Swift Foot (Usagi, Inaba only): Allows Inaba to increase her Move and Jump units by 2 and increases SPD by 100%, but she will not be able to use any skill for the next few turns. She can quickly traverse large maps with this skill.