In the Da Vinci Code Robert Langdon shows Sophie Neveu a picture of The Last Supper. Langdon mentions a hand with a knife. It looks like the hand floats in the air and it doesn't seem to be connected with one of the persons in the painting. Was this one of Leonardo da Vinci's 'hidden messages'?

There is indeed a hand that seems to float. You can see the hand with the knife clearly between Peter and Judas (the 4th disciple from the left).

But a study Leonardo da Vinci made before he made the actual painting reveals that the floating knife is not a hidden message. Peter holds the knife, with his wrist turned backwards. The study is on display in the Royal Collection of the Brittish Windsor Castle.