Jesus knew the hearts of the disciples and proclaimed that one of them would later betray them. But the disciples were perplexed. It's likely that they don't want to be the one to later betray Jesus.
One of the disciples leaned on Jesus's bosom (13:23). Another one leaned on Jesus's breast to ask Jesus who the betrayer is (13:24-25). It's implied from the closeness that both of them wanted to maintain their connection with Jesus and demonstrate their desire to maintain said connection.
However, remember back in 13:2 that the disciple who betrayed doesn't do any of these things that Peter did but rather had his heart focused on betraying Jesus. It's likely that Jesus knows who would betray Jesus because of his knowledge of the disciples' hearts.