Before one knows it; without one's awareness — expresses that something happened without noticing; essentially the same as '知らない間に' with a slightly old-fashioned tone.
気がつくと、知らぬ間に外は暗くなっていた。
When I noticed, it had gotten dark before I knew it.
知らぬ間に財布の中身が減っていたので、よく調べてみた。
I found that my wallet had lost money without my noticing, so I checked carefully.
彼は知らぬ間に会社で信頼を得ていた。
He had earned trust at the company before he even realized it.
知らぬ間に季節が変わっていたことに驚いた。
I was surprised to find that the seasons had changed before I noticed.