Usage patterns
〈事〉を上の空で聞くto listen to something with one's mind elsewhere
He was listening to the teacher's talk with his mind elsewhere.
上の空になるto become absent-minded, to zone out
I became absent-minded thinking about my girlfriend.
〈人〉は上の空で〈動詞〉someone does something while distracted or with their mind elsewhere
My mother answered distractedly, her mind elsewhere.