"A box without hinges, key, or lid, yet golden treasure inside is hid."

—Bilbo, in The Hobbit

Dragons, Trolls, Men, Orcs, Dwarves, and (presumably) Wargs all had penchants for accumulating treasure. In all likeliness, the items procured from plundering would be taken and hidden in holds and hidden vaults. Smaug's lair in the Lonely Mountain, Moria, and the troll den mentioned in The Hobbit would all serve as illustrations of this.