Charles Ellicott Commentary 2 Samuel 3:26

"And when Joab was come out from David, he sent messengers after Abner, and they brought him back from the well of Sirah: but David knew it not." — 2 Samuel 3:26 (ASV)

Sent messengers after Abner. —It is not clear whether this was done in Joab’s own name or in David’s name (though without David's knowledge), but in either case, Abner would readily assume that Joab's arrival had made further discussion desirable. His complete confidence in David is shown by his unsuspecting return.

The well of Sirah. —The only knowledge of this location is from the testimony of Josephus (Antiquities vii. 1, 5) that it was twenty stadia (two and a half miles) from Hebron; and there is still a spring and reservoir called Ain Sareh, slightly more than a mile north of the town. If this is correct, Abner must have just left David when Joab arrived.