。
Their whereabouts remain completely unknown.
Literal
Darkly [as-として] whereabouts [subject-が] not-known.
杳として (ようとして) is a literary adverbial from classical Chinese: 'darkly, obscurely.' 行方が知れない ('whereabouts are not known') is the standard expression for a missing person. The combination 杳として行方が知れない is a fixed literary phrase used in news, novels, and formal reports about missing persons. The rare kanji 杳 makes this distinctly literary — a learner might encounter it in news articles about disappearances.