Suppose you've been to Guau-Guau Island, which is on the starting island of Isla Santuario, in the Casas Cove area, in the middle of the water. You may have seen a Mysterious Chest that is locked, and even if you explore the island, it'll be for naught as the key needed is not here.
To unlock the Mysterious Chest, you need to find the Mysterious Key obtained during a Yaran Story Fetch Quest after completing Who's A Good Boy? The Yaran story is located in Madrugada in the Costa Del Mar area. To make this story appear, you have to complete a story quest Meet the Monteros.
If you already have the key, the chest is located on the island in the picture below.
To complete the Who's a Good Boy? story fast. The pupper wants to eat Crocodile meat, so if you have some already on you, put it into the dog bowl, and story complete.
To complete Fetch Quest, all you had to do is pet Chorizo, follow him around and collect the Mysterious Key. With the Key in hand, go back to Guau-Gaua Island and unlock the chest for a F*ck Anton Rifle.
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