unexplained

A death whose cause is not immediately known.

A situation where someone has vanished without a known reason.

原因不明の病気

An illness for which doctors cannot find a cause.

医師が原因を見つけられない病気。

An absence for which no reason is known.

未解決の謎

A mystery that has not been solved or understood.

解決も理解もされていない謎。