いつの間に

いつのまに【いつの間に】

●彼はいつの間にこんな技術を身につけたのだろう. When did he ever acquire such a skill?

・いつの間に抜け出したのか, 展覧会場から二人の姿は消えていた. They must have slipped away at some moment, because they were no longer to be seen in the exhibition hall.