In the dead of night, the king receives the startling news that the enemy camp has been abandoned. Yet, rather than rejoicing over this sudden rescue, he is consumed by suspicion. He views the enemy's disappearance not as a miraculous deliverance or a stroke of luck, but as a carefully orchestrated military ambush.
This reaction exposes the king's lack of faith. Even though the prophet had explicitly promised that salvation would arrive the very next day, the king completely ignores the message, choosing instead to attribute the event to enemy deception [מלבי״ם]. He outlines his theory regarding the trickery the Arameans have plotted [מצודת דוד]. He reasons that the enemy is fully aware of the desperate conditions inside the besieged city. Knowing that the Israelites are starving and eager to find food and spoils [רש״י, ביאור שטיינזלץ], the enemy has laid a trap.
According to the king, the Arameans merely pretended to flee [אברבנאל]. In reality, they have simply left their camp to hide nearby [ביאור שטיינזלץ]. The objective of this ambush is to wait for the city's residents to emerge. The king assumes that the severe hunger will force the people to rush out for food without pausing to investigate the empty camp [מצודת דוד]. Once the Israelites are outside, exposed and unprotected, the hidden Arameans will strike. They plan to capture the people alive and seize the perfect opportunity to enter the open city gates and conquer it entirely [אברבנאל, ביאור שטיינזלץ].