Charles Ellicott Commentary 2 Samuel 15:10

"But Absalom sent spies throughout all the tribes of Israel, saying, As soon as ye hear the sound of the trumpet, then ye shall say, Absalom is king in Hebron." — 2 Samuel 15:10 (ASV)

Sent spies. —These were agents who were to sound out the people in various parts of the land, and undoubtedly to communicate the conspiracy only secretly, and to those whom they found favorably disposed. They started from Jerusalem, perhaps at the same time as Absalom, or possibly had been sent out quietly, a few at a time, beforehand. The signal for the uprising was to be a messenger with a trumpet.