unbeknown

Adjective

• (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
    

Adverb

• without someone's knowledge
  • unbeknownst to me, she made all the arrangements
    

Related words

• Synonyms: 
  unbeknown(p) , unbeknownst(p) , unbeknownst