If someone arrives unannounced, they arrive suddenly and unexpectedly.

She appeared unannounced and took control of the meeting. 她突然出现并控制了会议。

not made publicly known

The Texan singer will be supported by two other bands, as yet unannounced. 这位得克萨斯歌手将由另外两支乐队伴奏，但还没公布是哪两支乐队。