In the promo, Lydia reads a treemail saying the idol is hidden in the woods. This probably means it'll be an ongoing search, and not some kind of challenge. As Mariner suggested, we may not see anyone finding the idol this week. The promo also shows Rafe commenting on the idol and saying, "We can use it at any tribal council." This pretty much confirms that the idol serves as a one-shot immunity that you can use at any point in the game. It should be interesting to see when it's used, and to see who it is who needs it so badly that they'll draw resentment toward themselves by finding it.