When you find a treasure chest in a level added by Goblin Menace, you get the message about finding the usual reward (poem or picture) and a secret map piece. Does finding all of them do anything? I've unlocked all 10 and haven't noticed anything special.
Which platform are you playing on? If you're playing on PC, they don't do anything yet, but Frozenbyte has stated that they will in the future. On Wii U, it unlocks the exclusive level.
UPDATE: Trine 2: Complete Story now uses them! Just like the Wii U version, they unlock the Dwarven Caverns level.