Stuart Hutchinson arrives at Krakauer's tent with odd news: Harris is missing. This is decidedly strange to Krakauer, as he had watched Harris descend to Camp Four with his own eyes.
After exploring the area around Camp Four, Krakauer notices "a single set of faint crampon tracks leading" (16.6) toward the edge of a cliff and assumes they belong to Harris.
When Krakauer wrote his initial article about the Everest disaster, this was the story he told about Harris's death. Unfortunately, it's dead wrong.
The man who Krakauer thought was Andy Harris was in fact Martin Adams—in their oxygen-deprivation stupor, they both hadn't realized that they were talking to one another. That still doesn't explain the footprints, though...