Head back to Castle Fynn and go straight to the throne room. You can either talk to Hilda or Gordon, then choose "Learn -> Cyclone". 

Leave the throne room to 1F, then take the stairs in the bottom-left corner to 3F. 
From there, go straight to enter the top-left room. Examine the mirror and select "Key Items -> Pendant". 

Leave the castle and enter Fynn Town.

Enter Paul's house in the bottom-left corner.
Talk to Paul then "Ask -> Cyclone".
Enter his bedroom, examine the wall in the top-right corner to open the secret passage.

Inside the secret room has 8 chests:

• C-179: Black Garb - Top row, 1st chest.
• C-180: Thief's Gloves - Top row, 2nd chest.
• C-181: Elixir - Top row, 3rd chest.
• C-182: Silver Cuirass - Top row, 4th chest.
• C-183: Ruby Cuirass - Bottom row, 1st chest.
• C-184: Gold Cuirass - Bottom row, 2nd chest.
• C-185: Gold Hairpin - Bottom row, 3rd chest.
• C-186: Blood Sword - Bottom row, 4th chest.

Leave the town.