Kommentar zu Divrej Hajamim II 18:9

dressed in [their] attire the royal raiment and seated in a threshing floor He is telling us that when the prophets prophesied before them, they were sitting outside the city, and spies of the king of Aram came there and heard that he [the prophet] informed the king of Israel that he alone would fall in battle. That is the reason that the king of Aram commanded his forces (v. 30): “Attack neither small nor great; only the king of Israel,” to fulfill the words of Micaiah. That is why it is written, “in a threshing floor,” for it was a place where spies could spy. If the meeting had been inside the city, the spies of the king of Aram would not have been able to infiltrate, for they would have detected that they were spies; but when they were outside the city, even if they [the Israelites] became aware of them, they could say, “We are going elsewhere, and the reason we drew near to the threshing floor is because of the great tumult and gathering of the armies here at the gate of the city; we were amazed, and turned off the road to see what was going on.” Consequently, the spies informed the king of Aram [what they had heard], who commanded to slay no one but the king of Israel. Otherwise, why did it enter his mind to command not to slay anyone of Israel, but the king? But he had spies who had heard Micaiah saying, (verses 16f.): “I saw all the Israelites scattered, etc. And the Lord said: These have no master.” This was a sign that the king would be slain, and that the Israelites would return each one to his house in peace. And they heard that he said, (v. 27): “If you return in peace...” The spies heard all and informed the king of Aram. The king of Aram then commanded to act in accordance with the word of Micaiah, and to slay only the king.