This is pretty interesting - while the actual painting in question is historically documented, I guess there really is not a definitive answer as to what state it what is when it was eventually covered over:

FLORENCE: One of the art world's great mysteries may be about to be solved: Did Leonardo Da Vinci paint a masterpiece that was lost in time, and does it still survive today? Italy's culture minister has given his go-ahead to explore behind a wall in Florence's renaissance town hall to see if it is hiding the 500-year-old painting Battle of Anghiari, sometimes known as the 'lost Leonardo'.

More here.