before one knows it; without noticing; unawares
Used when something happens or changes without the speaker being aware of it. Often implies a gradual or unnoticed process. Similar to 知らない間に, but うちに emphasizes the time frame during which the unnoticed event occurs.
See also: 知らない間に
Before I knew it, it had started raining.
He left the room without my noticing.
A set phrase composed of 知らない (negative of 知る, 'to know') + うちに ('while, during'). Literally 'while not knowing,' it has become a fixed expression for something happening without one's awareness.