I just was this story about the upcoming auction of one of the two Aston Martins used by James Bond. Apparently, the other was stolen on 1997 and hasn’t been seen in public since. (I’m sure it has been seen by the thief who jerks off over it every day.) I hadn’t heard that before.
So what other priceless objects have presumably disappeared into the vaults of millionaire thieves?
Both bikes used in “Easy Rider” were stolen and never seen again. It’s speculated that the thieves really didn’t know that the bikes they stole were the ones in the movie and they were just disassembled for the parts.
The biggest art theft in history occurred at the Isabella Steward Gardner museum in Boston in 1990. None of it has ever been seen since. I think it is kind of strange to steal famous art which is meant to be shown off but you can’t because it becomes so infamous after it is taken.
The Ark is missing… last I heard it was in some giant government warehouse. 
How about the remains of the original Peking Man? (Although they may merely have sunk.)
The body (assuming he’s dead) of D.B. Cooper and almost all of the ransom money he got by holding a plane hostage.
I guess the missing objects aren’t exactly priceless, but it’s a cool story.
Buzz Aldrin’s Omega Speedster wristwatch, which he wore during the historic Apollo 11 moonwalk, disappeared in 1971.
Or if you prefer - he left it somewhere on the sound stage when they were done filming. 
The Amber Room hasn’t been seen since the end of World War II.
How about President Clinton losing the codes for launching a nuclear strike?
I realize the codes were not stolen, but were merely lost. However, it fits the description of an object that went missing.
The Holy Grail
Lawrence lost the first draft, which was evidently very different, of The Seven Pillars of Wisdom in a cab.
The “From Hell” letter is missing.
Jimmy Hoffa.
Ali’s Gold Medal.
Waldo.
The Great Mogul Diamond has not been seen since the 17th century. At the time, it was the largest cut diamond in existence, and it has been called the most valuable object ever lost.
Don’t forget Jimi Hendrix losing the master tape to Axis: Bold as Love in a cab.
The original “Jules Rimet” World Cup trophy was stolen in 1983 and never recovered.
Amelia Earhart