Endon's Silver Mold (or Curious Silver Mold if it was retrieved before the quest) is a quest item for the Thieves' Guild quest "Silver Lining." Endon requests that the Dragonborn retrieve the mold for him as it was robbed from a Khajiit Caravan by Rigel Strong-Arm and her group of bandits on its way to Markarth.
Since it is a quest item, it can not be dropped or stored in a container.
It is found in the treasure room at the end of Pinewatch Bandit's Sanctuary, surrounded by various gems, ingots, ore, gold, a coin purse, and two Silver Candlesticks. The quest does not need to be active for the item to appear; it can be obtained any time before the quest. Even if it is taken before starting the quest, there will not be any problems with completing it (unlike some other quests, like "The Pursuit").