Indicates that something happened without one's noticing or before one realized it; 'before one knows it'.
知らない間に雨が降っていた。
It had rained before I knew it.
気づいたら、知らない間に庭の桜が満開になっていた。
When I noticed, the cherry trees in the garden had become full bloom before I knew it.
スマホを見ていると、知らない間に時間が過ぎてしまう。
When I look at my smartphone, time passes without my noticing.
知らない間に彼は会社を辞めていたらしい。
It seems he had quit the company before anyone knew.