Treasure Hunter is a challenge in Dead Island. It requires you to open a certain amount of lootable containers. Lootable containers are any suitcase, cabinet, basket, etc. that can be opened. This challenge is easier unlocked through natural progression as you'll be doing a lot of looting.
Level 1 is 100 containers, and the reward is 2,000 XP. Level 2 is 250 containers. Level 3 is 500 containers, and the reward is 15,000 XP. level 4 is 1,000 containers, and the reward is 80,000 XP.