Surely it is not I, Rabbi?
The disciples never used the title, "Rabbi," when addressing Jesus. He told them not to. Others who were not close to him, called him Rabbi, e.i., Pharisees and Sadducees.
So when Judas asks "Surely it is not I, Rabbi?" everyone at the table must have perked up. Something's amiss.
Yes, this is a foreshadowing of Judas' separation from the disciples.