[1] : WordNet (r) 2.0

unannounced
     adj : without warning or announcement; "they arrived unannounced";
           "a totally unheralded telegram that his daughter...died
           last night"- m.a.d.howe syn: unheralded, unpredicted
see also:
unheralded unpredicted 

[2] : Moby Thesaurus II by Grady Ward, 1.0

18 moby thesaurus words for "unannounced":
   extraordinary, improbable, more than expected, out-of-the-way,
   past expectation, sudden, unanticipated, undivined, unexpected,
   unforeseeable, unforeseen, unguessed, unheralded, unhoped for,
   unpredictable, unpredicted, unprepared for, without warning

Unannounced >> U Words
Unannounced, definition of term: Unannounced
unannounced_pag1.html