What does Xiao Lan mean by writing "XXXX" on the short message signature of an episode of Detective Conan?

I. Introduction

The TV cartoon Detective Conan is adapted from the comic work of the same name created by G?sh? Aoyama, produced by TMS Entertainment Company (formerly Tokyo Film News Agency) and premiered on Yomiuri TV on June 8th, 1996.

Second, the introduction of characters

Xiaolan's father was a policeman and an old friend of Mu Mu Police Department. Now it's open to the Maori detective agency. Thanks to Conan's famous reasoning, he is famous for Sleeping Little Five Lang. Through Xiaolan's efforts, she often meets Fei Yingli, her separated wife. Because of her lascivious side, it is difficult to get back together honestly.

She looks like a child, but she is actually a member of the black organization, the developer of the new poison (APTX4869), code-named Shirley. My sister was killed, betrayed the organization, took drugs and fled, and became a primary school student. Now I am hiding in my home in Agasa Hiroshi, studying in Didan Primary School with Conan, and studying the antidote on the other side. Conan is the only person in the world who can be in the same situation as her.

A high school detective who worships Sherlock Holmes. Born on May 4th, 17 years old. Class B, Grade Two, Didan High School. When I was a high school student in grade 1, I solved a murder case on the plane to Los Angeles, where I met police officer Mu Mu and others. Since then, he has become an active high school detective. His specialty is football, and his thinking is clear and outstanding.

When I went to the amusement park (tropical paradise) with Xiaolan, my childhood friend, I met the trading scene of men in black and was organized to pour the poison "APTX4869" under development. Since then, there has been no news. After poisoning, it became the body of a primary school student. )