unaccounted for

またしても日本人が海外でテロの被害者になりました。July 2, 2016付けのThe Japan Times Onlineの記事冒頭を引用します。
All the seven Japanese citizens who were previously unaccounted for after a 12-hour hostage crisis in Dhaka were found and pronounced dead at a local hospital, Chief Cabinet Secretary Yoshihide Suga said at a news conference in Tokyo late Saturday night.
"unaccounted for" は知っている "account for" の反対/否定する意味とは異なる様です。辞書で確認します。
・Oxford English Dictionary: Not included in (an account or calculation) through being lost or disregarded: a substantial amount of money is unaccounted for
・Merriam-Webster's Online Dictionary: used to say that what happened to someone or something is not known: Many people were unaccounted for after the disaster.: A great deal of money remains unaccounted for.
こんな意味で使われるのですね。