いつの間にか

いつのまにか【いつの間にか】

before one knows [is aware]; while one was not aware of it; unnoticed; without one's knowing [realizing] it; before one knew what was happening; at some time or other.

●冬休みもいつの間にか過ぎてしまった. Winter vacation has ended all too soon.

・台所にいつの間にか犬が入っていた. Somehow, a dog had found its way into our kitchen. | There was a dog in our kitchen! <▲> 無理に「いつの間にか」を加えないほうが英語としては自然.

・彼は先ほどまでみんなとにぎやかに騒いでいたが, いつの間にかいなくなった. He was here just a moment ago, partying [celebrating] with the rest of them, but now he's gone.

・ユリがいつの間にか咲きだした. The lilies were already blooming.

・いつの間にか秋が来た. I can't believe it's already autumn.

・いつの間にか眠ってしまっていた. At some time or other I had dozed off.

・いつの間にか白髪がこんなに増えた. When did my hair get so gray?