Charles Ellicott Commentary 2 Samuel 15:27

"The king said also unto Zadok the priest, Art thou [not] a seer? return into the city in peace, and your two sons with you, Ahimaaz thy son, and Jonathan the son of Abiathar." — 2 Samuel 15:27 (ASV)

Are you not a seer?—The Hebrew is difficult and must be translated either: Are you a seer? or, with a very slight change in a vowel, as an address, You seer. Zadok is called this because he was now, in some way, to fulfill the office of a prophet in guiding David’s course and also in making known to him the events taking place in Jerusalem that would show God’s will for him.

Nothing is said in any part of this narrative about Nathan and Gad, both of whom were certainly still living (2 Samuel 24:11; 2 Samuel 24:13–14; 1 Kings 1:11).

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