Here is the full list of all 23 Hidden: On the trail of the Ancients achievements.
Examine every painting and picture in the boarding house.
Find a way into what used to be Adriano Scopelli's room.
Fix the music box.
«I was in the front during the beginning of the Great War...»
Find and collect information regarding Scopelli's diagram.
Find the artifact which Adriano Scopelli mentions in his letters.
Find and take a picture of the strange idol.
Complete your mission at the boarding house.
Gathers information from the expedition , see what happened.
Find the device mentioned by Dr. Laura Garmendia.
Get rid of the creature.
Find and protect the valuable object mentioned by Eneko.
Completed the first part: "On the trail of the Ancients".
Find and examine all books.
Read all books and notes.
Collect all findings.
Finish the game without using the hints system.