How did the police station know when the petition came back?

The police station will record the process of letters and visits. According to Baidu's legal information, letters and visits refer to a system in which individual citizens or groups contact with state political parties, governments and other institutions or personnel in charge of letters and visits in various forms, such as letters, mails, visits, telephone calls, faxes and short messages, so as to reflect the situation, express their opinions and request to solve problems. Petition-related institutions or personnel will handle it in a certain way. A normal petition will leave a petition record at the police station, but this record is not a criminal record.