After finishing reading the Third Man Factor, the very next book I picked up was Great Heart: The History of a Labrador Adventure by Davidson and Rugge. Lo and behold, one of the expedition members on the 1903 Hubbard expedition to find Lake Michikamau, experienced a Third Man in the manner and circumstances that fits the descriptions in Geiger's book perfectly.

Leonidas Hubbard, Jr., the expedition leader, had become too weak from starvation to go any further and was left behind in a tent while the two other members, near collapse form hunger themselves, went forward to retrieve a cache of flour. Once they had found it, one went on to find help, while the other, Dillon Wallace, was tasked with taking the flour back to Hubbard. In the process of Wallace's hiking back, the October weather in Labrador turned stormy, and he soon lost his bearings in a newly-snow covered landscape, which covered up landmarks and compounded Wallace's near smoke blindness (from lying beside a soggy fire a few days prior). Having determined at some point that he would not be able to find the tent (as it happened his tracks in the snow showed he had been only a few hundred feet from the now-buried tent, with Hubbard's corpse inside), Wallace decided he had no choice but to turn back and try to follow the river in the hopes that a rescue party would be coming upriver.

As he struggled through a blizzard, he was close to giving up, when suddenly he felt the presence of his wife, who had died only a few years before, and her voice telling him to keep on. He did not feel it was strange, but took comfort in it. For the next few days, as he wandered through the wilderness, the presence of his late wife, pushing him, guiding him, directing him kept him going, until at last he was found by a party of trappers organized as a rescue effort.

The account in the the book I read is worth reading, as it fits the experiences in Geiger's book perfectly. Wallace himself wrote an account of the expedition entitled "The Lure of the Labrador Wild," which I believe contains his firsthand account of it as well.