Saul probably supposed that not only Jonathan, but also some considerable number of his army, was gone, and that by them that slaughter was made.
Related Commentaries of 1 Samuel 14:17
1 Samuel 14:17
17 Then said Saul unto the people that were with him, Number now, and see who is gone from us. And when they had numbered, behold, Jonathan and his armourbearer were not there.