The Fourth Crewmember

The robot listened for the homing signal.
Nothing. No sign of the crew, either.
Looking down at the pile of rocks, it decided to investigate.
An hour later, all of the rocks were removed, revealing three battered corpses.
The robot’s visual records were corrupted, so it took DNA samples and did a quick analysis.
TRAVIS. BLAKE. AL-MAJD.
All matched with the crew.
Placing the bodies back in the grave, the robot stacked the rocks back up.
The robot snapped a photograph for-
ERROR
It rebooted, and the robot listened for the homing signal.
Nothing. No sign of the crew, either…