The Battle of Anghiari
This paintings is usually referred to as Leonardo's Lost painting or "the Lost Leonardo." It was painted around 1505 and represents the battle of Anghiari that took place in 1440. Researchers believe that it is in the in the "Salone dei Cinquecento (Hall of the Five Hundred) in the Palazzo Vecchio" and hidden under another fresco painted by Leonardo's first biographer Vasari. When da Vinci was given the commission in 1504 Michelangelo was also hired to paint another mural in the same room and was the only time they worked together. Both of these murals were either 'lost' or covered later. There is controversy as to whether Leonardo's painting is still there and why it was covered up. Researchers took samples and found paint that was similar to the type used for the Mona Lisa and St. John the baptist.
Since this painting was documented and copies were made of it, by Vasari especially, it's strange that Vasari would have covered it over? In my book I assert that the title "Mona Lisa" which was first coined by Vasari - was used intentionally to contain the anagram "Mon Salai." Since it didn't go by this title until after Leonardo died and was used by Vasari first - I found it problematic that it could have been the title da Vinci himself intended. Why wouldn't he have just titled the painting "Mona Lisa" himself? It wasn't until I read about this Vasari/ Battle of Anghiari correlation that I could find a direct connection. If Vasari saw this painting and then "covered it up" (Literally) it seems that there would have to be a reason. There are also some strange anomalies in his biography of Leonardo that also allude to him providing clues to da Vinci's works. I'm unsure on the details but there seems to be an extended collaboration between the two which would have had to been facilitated by an intermediary person(s) or directions left behind by Leonardo himself. I think that Leonardo left behind a secretive set of directions or clues to his companions/ collaborators that would set up and expand on the "puzzles and riddles" embedded in his works. Kinda like he said "After I die make sure X happens" or that his last wishes were willed to certain people that would place other pieces of the puzzles that he couldn't during his own life time.