Before one knows it; without one's noticing — something that happens while one is unaware.
寝ている間に知らないうちに雨が降り始めていた。
While I was asleep, it started raining before I knew it.
気づいたら知らないうちに指輪がなくなっていた。
When I noticed, the ring had gone missing without my realizing it.
知らないうちに語彙が増えているのは、毎日少しでも学ぶ積み重ねのおかげだ。
That your vocabulary grows before you know it is thanks to daily small efforts adding up.