WordNet (r) 3.0 (2006):

unannounced
    adj 1: without warning or announcement; "they arrived
           unannounced"; "a totally unheralded telegram that his
           daughter...died last night"- M.A.D.Howe [syn:
           unannounced, unheralded, unpredicted]

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