Like a lot of my other ideas, this one could either be truly awesome, or fail miserably when tried in practice.
The characters are all intelligent robots, dong something far away from Earth, perhaps setting the ground work for a colony on Titan, with no direct human supervision, and no little or no contact with Earth.
After a while, they realize that they have not had any updates, or requests for information from Earth for a long time, and their queries go unanswered.
Being independent AIs, they decide to investigate, and use some of the construction machinery to build a way to get back to Earth. There they find that humanity has disappeared.
The players are playing the robots, trying to figure out what happened to their human masters, as they slowly piece together the puzzle, the players will play through brief vignettes of the humans in the days leading to whatever happened to them.