Gu Nanyan wasn't keeping track of Lu Beicheng's whereabouts, let alone Shen Weiyi's.
It was the night that Shen Weiyi went to Yulin Bay when Gu Nanyan received an anonymous email filled with photos of Shen Weiyi at Yulin Bay.
Gu Nanyan looked at them but treated them as if she hadn't seen anything, understanding full well that someone deliberately wanted her to know. Hence, they only sent the photos of Shen Weiyi arriving at Yulin Bay, not leaving, clearly aiming to create a misunderstanding.
However, she couldn't care less about Lu Beicheng's love life, so she did not bring up the matter during the days after receiving the photos.
She also understood clearly that Lu Beicheng couldn't possibly have something going on with Shen Weiyi at this time, given that he had been devoting his time and attention to her and the little treasure.
The reason she brought it up now was because Lu Beicheng had just made a sarcastic remark towards her, so she was intentionally prodding him.