It's a trope which has existed possibly for as long as writing has existed: The protagonist finds an abandoned ship, hut or similar. In it, is a log or record of some kind. They read the final entry in the log, and it either explains outright what happened to the former owner, or else it provides clues to the owner's fate, which the protagonist then decides to follow up on.
Subtrope of Apocalyptic Log
, but Doomsday Log applies when only the last entry or the last few entries of the log are important to the plot.
Can be a subtrope of MacGuffin
when the log is carried around by the protagonist and is used to drive the rest of the plot.
If the person is only missing instead of dead, the missing person may become a Living MacGuffin