The Stranger has had encounters with other humans – as well as other creatures. They are building a transmitter.
Document Text:
Day 175: There are others here, trapped like me. I avoid them. No time for their madness or sorrow. They usually get picked off before long, anyway. We are low on the food chain here.
Creatures prowl this place, so I avoid them, too. They can be killed, but gunshots attract more, so discretion better part of valor.
Still have food stockpiled, but cannot last forever. Eventually I will be forced to hunt. And nothing looks remotely edible. Yet.