Etymology
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unannounced (adj.)

1775, from un- (1) "not" + past participle of announce (v.).

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Definitions of unannounced

unannounced (adj.)
without warning or announcement; "a totally unheralded telegram that his daughter...died last night"- M.A.D.Howe;
they arrived unannounced
Synonyms: unheralded / unpredicted
From wordnet.princeton.edu