2 Samuel 15:27

Art not thou a seer? — The Hebrew is difficult, and must be translated either. Art thou a seer? or, with a very slight change in a vowel, as an address, Thou seer. Zadok is so called because he was now in some sort to fulfil the office of a prophet in guiding David’s course, and also in making known to him the events taking place in Jerusalem which would show God’s will concerning him. Nothing is said in any part of this narrative of Nathan and Gad, both of whom were certainly still living (2 Samuel 24:11; 2 Samuel 24:13; 1 Kings 1:11).

Your two sons with you. — Zadok only has been mentioned, and probably Abiathar was not present at the moment, but David shows by this way of speaking that he means to address them both.