What is the first sign foreshadows trouble for campers in Duffys Jacket?

In "Duffy's Jacket," the first sign that foreshadows trouble for the campers is the discovery of the abandoned campfire site, which suggests that they are not alone in the woods. This unsettling realization heightens the tension and sets the stage for the eerie events that follow. Additionally, the ominous atmosphere and the children's growing sense of unease contribute to the sense that danger is lurking nearby.