Note: This quest takes priority over receiving the Trust: Trion spell. If you are trying to receive the trust spell, you may have to click on the door multiple times until you receive the proper cutscene for it.
You may finish the quest as any job at this point.
After the cutscene, head to Southern San d'Oria and talk to Vemalpeau (M-7), who lives inside the small house nearest Lion Square, on the bottom floor.
Then go to Lion Spring's Tavern and talk to Najjar.
To the right is a map of the coffer locations. Basically, the key dropping monsters are in the four corners and the tunnels leading to the four corners. To get to these corners, you need to drop down into four different holes. However, the northwest hole will only lead to the northwest corner, and once you drop, you must make your way back up to get to a different hole.
Proceed upstairs in Vemalpeau's house. Immediately to your left, there are three spots that can be examined. Click on the painting on the wall above the first spot and select the option to "look behind the frame" and then "open and examine" to obtain a Strange Sheet of Paper.
(optional) Return to Vemalpeau who mentions his son's commanding officer.
Go to "Helbort's Blades" (K-7) and talk to Exoroche, who tells you to go to Davoi.
Although the quest is technically over after talking with Trion, the Knight's Confession will remain in your inventory until speaking with Vemalpeau at (M-7) one last time. Giving him his son's confession serves as a bit of closure, though this is entirely optional.
Trion (Prince Royal's Room, Chateau d'Oraguille)
Knights whisper of a traitor in their midst. An outsider is needed to investigate.
SPOILER WARNING: Click here for additional details about this story line.