Ok....maybe The Maltese Falcon isn't exactly a lost Pirate Treasure, but in the introduction to the 1941 movie was this:
In 1530 the Knight Templars of Malta, paid tribute to Charles V of Spain, by sending him a Golden Falcon encrusted from beak to claw with rarest jewels- but pirates seized the galley carrying this priceless token and the fate of the Maltese Falcon remains a mystery to this day.
So it sort of is....
Although a movie, the movie prop of the Maltese Falcon used in the movie of same name, has become quite valuable. It is unclear how many exist. It is thought maybe only 6 resin Falcons and 2 metal falcons were created for the film. One of the metal falcons selling in 2013 for over 4 million dollars. Others, even the resin, have sold for hefty prices as well.
However, no one knows for sure, and other Maltese Falcons could be out there to find.