WHERE IT STARTS:
The Treasure Hunt starts at a house by the beach at Siniestra, AGUDA CLIFFS, COSTA DEL MAR.
THE NOTE SAYS:
I did it. I caught him.
He has tormented me for TOO LONG. Putting my house up on stilts didn't stop him. Building the fence didn't stop him. BUT I FINALLY DID.
Please STAY CLEAR OF MY HOUSE. I've gone to get some gasolina from Luis. I'm going to end this once and for all.
He might be a mongoose, but to me he'll always be La Comedreja.
WHAT TO DO:
Written Guide below the Walkthrough Video
After reading the Treasure Note, follow the fencing on the right side until you see a gate with a lock. You can shoot that lock to get inside.
Then, you may want to search the house before you go to the shed. There is a HIDDEN HISTORIES COLLECTIBLE (Art and Death #1) inside on the kitchen table.
According to the letter on the table, the Mongoose has swallowed the chest key.
Go down to the shed. The key to the shed is right in front of it.
Kill the Mongoose and you can open the chest inside the house.