in a private place with no one else present and without other people knowing

The negotiators were meeting in secret for several months before the peace agreement was made public. 在和平协议公布以前，谈判秘密地进行了几个月。

only in someone's thoughts, without telling other people

He says he loathes her, but I think in secret he really likes her. 他说他恨她，但我认为他心里其实是喜欢她的。