Why does Alan leaves a cross at his friend's window?

This is actually in chapter 21. Alan made a cross out of wood in the tradition of the fiery cross, a signal for the gathering of his clan. He attached the silver button he had given David along with sprigs of birch and fir. Then, Alan explained that he would leave the cross outside the window of John Breck Maccol, a friend of his. John would recognize the button. Also, he would be able to discern that the two sprigs identified in which part of the heather Alan was hiding.