Krakauker introduces the circumstances surrounding McCandless' death early in chapter 2. McCandless is living on a bus in the remote Alaskan wilderness, and he leaves a note, hoping someone will find him:
S.O.S. I need your help. I am injured, near death, and too weak to hike out of here. I am all alone, this is no joke. In the name of God, please remain to save me. I am out collecting berries close by and shall return this evening. Thank you, Chris McCandless. August?" (p. 12)
Weeks later, hunters approach the bus, wanting to use it during moose season. They find the body in the bus and call the bus. At this point, they have no idea who he is or how he died. All they know is that it was difficult to get to the bus; they used rigged vehicles to get across the river, then continued on all-terrain vehicles. They know there must be more to this story than they are seeing.
If McCandless had not gotten sick until a couple of weeks later, he may have made it out alive. No one was around when he got sick because it was not moose season yet. The reason people found the body was because moose season had opened.