The most famous hidden or lost treasure I can think of right now it's the Spanish galleon which sank in near the Isle of Mull (Inner Hebrides) in 1588. This ship belonged to Castillian squadron of the Armada and fled north after the Spanish fleet was defeated by Howard, trying to get home by the long route.
The Royal Navy and the Duke of Argyll did some searches in the area in 1950 and found a few items belonging to a XVI century Spanish ship confiming local traditions: a few cannons, an anchor, a few human bones and a handful of Spanish coins. They concluded that the wreck had broken up and the wooden hull disintegrated with time. Bad weather and poor diving conditions meant the searches were called out after three weeks.
Local traditions give the galleon the name San Juan de Sicilia. There was a ship of this name in the Armada but historians have also suggested that it could be the Duque de Florencia. Neither ship was captured by the English and both failed to return home, so it may well be. The ship threw anchor in front of Tobermory and the Spanish captain asked the MacLeans of Duart, the clan to which the island belonged, permission to carry out repairs in the bay. This was duly granted (the Spaniards were good paying customers) but at the same time a message was sent to Edinburgh asking for instructions.
The English ambassador immediately informed Her Majesty and was allegedly instructed to destroy the ship. A spy by the name of John Smollet managed to sneak on board in disguise and set fire to a slow-burning match in the powder magazine. As you may expect he escaped minutes before the ship burst in flames.
Locals held that the ship was carrying a large treasure of silver and gold and a few crude attempts were made to recover it in the past four centuries. All were notable for their lack of success.
Whatever the ship actually carried large sums of money (pay for the crews of the Armada, for example) or the locals were simply impressed by the comparative wealth of the Spaniards we'll never know for sure.