"Hehe, Sonja is so curious to see what's in the chest of mine. Little does she know I hid the key near her little goblin campsite in the arena."
When his mind is read, he reveals that he left a key to his chest inside the goblin camp, which is where the player is sent by Sonja, who ironically is the person Edmund is hiding it from.
The key is on a rock next to the bonfire in the Goblin camp. It unlocks the chest next to Tom's house, on the left behind the bushes.