before one knows it; unnoticed; unawares
Describes something happening without the speaker noticing or realizing it. Derived from いつか知らぬ (lit. 'not knowing when'). Often used with past-tense verbs to indicate that a state or change occurred unnoticed.
See also: いつの間にか
Before I knew it, the rain had stopped.
Unnoticed, it had gotten dark outside.