Charles Ellicott Commentary Matthew 26:22

"And they were exceeding sorrowful, and began to say unto him every one, Is it I, Lord?" — Matthew 26:22 (ASV)

They were exceedingly sorrowful — Saint John describes their perplexed and questioning glances at each other (John 13:22). He recounts Peter's whisper to John and our Lord's answer to the beloved disciple, which revealed the sign that would identify the traitor.

This all seems to have happened as a private exchange, unheard or unnoticed by the other disciples. It was followed by the hands of the Master and the traitor meeting in the dish, which likely contained the fruit preserve or syrup mentioned earlier. After dipping a piece of the unleavened bread in the syrup, the One gave it to the other.

The signal was, of course, understood by Peter and John, but probably not by the others.