John Gill Commentary 1 Samuel 26:4

"David therefore sent out spies, and understood that Saul was come of a certainty." — 1 Samuel 26:4 (ASV)

David therefore sent out spies
To observe if he was coming or come, and where he was, that he might not be surprised by him; for though David knew the Lord was and would be his protection, he thought proper to be upon his guard, and to make use of means for his safety:

and understood that Saul was come in very deed ;
that he was most certainly come, and come to some certain place; which he himself went to reconnoitre, as in (1 Samuel 26:5) .