What does 2 Samuel 3:23 mean?

When Joab and his whole army arrived, people told him, 'Abner, son of Ner, came to see the king, and the king let him go in peace.' 2 Samuel 3:23 - Modern Text Bible

(Joab hears that Abner, his rival, was just here and left unharmed, which surprises and angers him.)