Behold: see 1.2, 18; and 3.12. The word Behold in this case seems to draw attention not to Abner’s visit but to what Joab is about to observe concerning that visit.

Abner came to you: implied in this statement is the fact that Joab has been informed about Abner’s visit. In some languages it may be more natural to say “I know [or, learned] that Abner was here….”

So that he is gone: this repetition of the idea of Abner’s departure seems to be intended to emphasize the fact that he is no longer within the grasp of Joab, who was seeking revenge. The thought is well expressed by New Jewish Publication Society’s Tanakh: “now he has gotten away!” New Revised Standard Version has clarified the Revised Standard Version by saying “so that he got away.”

Quoted with permission from Omanson, Roger L. and Ellington, John E. A Handbook on the First and Second Books of Samuel, Volume 2. (UBS Helps for Translators). New York: UBS, 2001. For this and other handbooks for translators see here .