If the content of WeChat involves criminal cases, especially the investigation clues of important criminal cases, the public security organs at or above the county level may take technical investigation measures against the mobile phone number. This method can track the location of the mobile phone (it is useless to turn it off, but it can also be tracked), monitor calls and intercept WeChat.
Criminal procedure law
Article 148 After putting a case on file, the public security organ may, according to the needs of investigating crimes and through strict approval procedures, take technical investigation measures against crimes endangering national security, terrorist activities, organized crimes of underworld nature, major drug crimes or other crimes seriously endangering society.