Each day, after waking up, Rin checks to see if she's gotten any messages. At the beginning of the short, she hasn't gotten any messages in over 2,539 days. That's roughly seven years without any form of communication with another living thing. Her sigh sounds resigned, like she's gotten used to her solitary existence.
The flashback sequence shows that Rin's father saved his beloved daughter from becoming a casualty by a planetary collision. It's reasonable to assume that he, the whole of humanity, and even the Earth itself were wiped out. As if Rin wasn't lonely enough...
