whereabouts unknown; untraceable; long lost
Used when a person's location or status cannot be confirmed, often after a disaster, disappearance, or long absence.
After the earthquake, he went missing.
A friend who had been untraceable for ten years contacted me.
行方不明 also means 'missing' but focuses on the physical whereabouts being unknown, often used in police or news contexts. 消息不明 emphasizes the lack of any communication or news, and can apply even if the location is roughly known.
Compound of 消息 (news, tidings) and 不明 (unknown, unclear).