A chalk mark on a lamppost, a certain color of sheet on a clothes lines or even a cryptic message in the classified section of a newspaper are all possible signals.

• 路灯柱上的粉笔痕迹，挂在晾衣绳上的特定颜色的纸甚至报纸上登出的加密信息都是有可能当作是信号的东西。