The Ziphites came to Saul to Gibeah, saying, Doesn't David hide himself in the hill of Hachilah, which is before the desert?
"David sent out spies and learned of a certainty that Saul had come"  (1 Samuel 26:4).  David's reluctance to believe that Saul had actually come out with an army to hunt him on this occasion, and which he would not believe until his spies confirmed it, proves the truth of the previous narrative.  After all that Saul had said then, David could hardly believe the reality of this additional attack.