can somebody remind me how the male lead figured out the the protagonist is female?

He saw her in the bath when he went back to the room because he forgot his phone. That made him suspicious of her gender, then he got an email from an investigation saying Lu Yining had a sister, then he knocked her out with sleeping drugs and found her chest bindings and put 2 & 2 together that the person he fell for was Lu Yining's sister, Lu Ningning. He still doesn't know that she's the stepson of the Smith family

LMAO WHAT that's crazy how do I not remember Σ( ￣□￣||)

do you know around which chapter? estimating haha