Netnt’s answer is still accurate. It seems that these so-called experts are not very professional, or do they have other purposes? Not to mention other things, even if you think about it from the perspective of social security, if the public security organ wants to detain someone, should they just let this person escape because the person or his family member refuses to sign?
The legal basis is as follows: "Criminal Procedure Law of the People's Republic of China": Article 61 Public security organs may detain current criminals or major suspects in advance if any of the following circumstances apply:
(1) The person is preparing to commit a crime, commits a crime, or is raped immediately after committing a crime;
(2) The victim or someone who witnessed the crime identifies him with the crime;
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(3) Evidence of a crime is found around him or in his residence;
(4) He attempts to commit suicide, escapes or is at large after committing a crime;
(5) There is destruction , forging evidence or possibly colluding with confessions;
(6) Not telling the real name, address, and identity is unknown;
(7) Committing crimes on the run, committing crimes multiple times, or committing serious crimes in groups Suspicious.
Article 64 When detaining a person, the public security organ must produce a detention certificate.
After detention, unless there are circumstances that hinder the investigation or cannot be notified, the reason for the detention and the place of detention shall be notified to the detained person's family or his unit within 24 hours.
Article 65: The public security organs shall interrogate a detained person within twenty-four hours after being detained. When it is found that a person should not be detained, he must be released immediately and issued a release certificate. For those who need to be arrested but the evidence is not sufficient, they can be released on bail pending trial or placed under residential surveillance.
In addition, as heraldboy said, it is very feasible to ask an acquaintance in the detention agency (maybe the police or the prosecutor's office, I don't know your actual situation) to ask what the case was for. Or just ask the detention authority directly, but that would be more troublesome.