1 Samuel 23:7
And it was told Saul that David was come to Keilah. And Saul said, God hath delivered him into mine hand; for he is shut in, by entering into a town that hath gates and bars. And it was told Saul that David was come to Keilah. And Saul said, God hath delivered him into mine hand; for he is shut in, by entering into a town that hath gates and bars.
Saul was told that David had come to Keilah. He took the news as an opportunity to capture him. He concluded that David was shut in and trapped by having entered a fortified town with gates and bars.