Open English Wordnet
Adjectives
(s)unbeknown, unbeknownst(usually used with `to') occurring or existing without the knowledge of“a crisis unbeknown to me”, “she had been ill for months, unbeknownst to the family”
Adverbs
(r)unbeknown, unbeknownstwithout someone's knowledge“unbeknownst to me, she made all the arrangements”