のに気付いた

noticed that

→ の

phrase Rare #36,042

noticed that の turns the preceding clause into something noticed; 気付いた is the past form of 気付く, “to notice.”

JA: 彼女が帰ってきたのに気付いた。

EN: I noticed that she had come back.

トムは、酔っぱらいが道に横になってるのに気付いた。

Tom noticed a drunk lying in the street.

コンサート中、数人が寝ているのに気付いた。

During the concert, I noticed that several people were sleeping.

いつ財布がないのに気付いたの？

When did you realize your wallet was missing?

私は彼が立ち止まっていたのに気付いた。

I noticed that he had stopped.