The Crown Prince Mansion
Zhu Faye wasn't the only person to receive a letter from an anonymous person, two of such letters were sent to the Crown Prince's mansion. One was addressed to the Crown Prince while the other was addressed to Zhu Fenfang. No matter how much Zhu Faye tried to find out who it came from but found nothing,
Whoever sent him the letter hid well, although he had a few suspicions because of the content of the letter. While he couldn't find where or who the letter came from he discovered that two of such letters were sent to the Crown Prince's mansion. This pricked his interest since instead of the Crown Prince and Crown Princess consort being the recipient of the letter.
He was surprised when the other recipient apart from the Crown Prince was Zhu Fenfang. This got him investigating his younger sister, he was curious about what the sender found in her.