After what happened in the morning, Trixie obviously felt Frank was acting differently.  He seemed to avoid her deliberately and even asked other colleagues to show her the documents, making Trixie more suspicious of his relationship with the club.

She even speculated whether Frank had joined "taboo love."

However, since Trixie joined the company, she had never heard of Frank's girlfriend, let alone his wife, so he had no conditions to enter the club.

Given the situation, ...