The money bag has a tracking device, and Chigurh is carrying the unit that tracks it. At one point, as Chigurh returns to a hotel where he previously had a shoot-out with the man he is hunting, the transponder unit begins to beep. McCarthy then gives us a glimpse of Chigurh's cruelly collected, cognitive processes at work:
He could think of no reason for the transponder sending unit to be in the hotel. He ruled out Moss because he thought Moss was almost certainly dead. That left the police. Or some agent of the Matacumbe Petroleum Group.
Who must think that he thought that they thought that he thought they were very dumb. He thought about that.—McCarthy, No Country for Old Men, p. 171