Jamieson, Fausset and Brown on 2 Samuel 15:10

10. Absalom sent spies throughout all the tribes of Israel--These emissaries were to sound the inclination of the people, to further the interests of Absalom, and exhort all the adherents of his party to be in readiness to join his standard as soon as they should hear that he had been proclaimed king. As the summons was to be made by the sound of trumpets, it is probable that care had been taken to have trumpeters stationed on the heights, and at convenient stations--a mode of announcement that would soon spread the news over all the country of his inauguration to the throne.