secretly       
adverb uk/ˈsiː.krət.li/ us/ˈsiː.krət.li/
B1 秘密地
in a way that people do not know or are not told about
  She said she didn't care about it, but I believe she was secretly delighted.
 He was convicted on the evidence of secretly recorded phone conversations. 他是根据秘密录制的电话谈话证据被定罪的。 
 See: secret
 - More examples
 - The two parties met secretly and signed a joint declaration.
 - I think we all secretly wished we would be caught.
 - The video had been shot secretly inside the factory.