שמואל ב, פרק י״ח, פסוק כ״ה

A watchman alerts the king to an approaching figure, prompting a quick assessment based on simple military logic. The king deduces that a man running alone must be a messenger carrying news, meaning there is no immediate cause for alarm [ביאור שטיינזלץ].

This conclusion comes from the practical reality of war. Soldiers retreating from a battlefield after a defeat do not flee as individuals; rather, they escape together in a massive, panicked retreat [מצודת דוד, ביאור שטיינזלץ]. Because the approaching man is by himself, he is clearly a purposeful messenger who simply continues to advance, steadily closing the distance toward the king [ביאור שטיינזלץ].