Song Ze, a volunteer who worked with the dissident rights lawyer Xu Zhiyong’s Open Constitution Initiative to help provide humanitarian aid to petitioners, was detained and later switched to "residential surveillance" in June. Since then, his whereabouts have not been revealed by police. Lawyer Xiao Guozhen recalls Song's earlier actions promoting human rights that could have possibly angered the government. From The New York Times:
At the end of December, on the day of the Laba Rice Congee Festival, when Chinese families typically eat congee, a type of rice porridge, Mr. Song wanted to deliver some congee to the petitioners. I told him that if he distributed it in the evening, I could go with him. But he said that in accordance with Northern custom, the congee should be eaten at lunchtime and so Mr. Song did it on his own. On his way, he was stopped by the police, and the porridge was confiscated. On the day of the Lantern Festival, which marked the end of the annual Chinese New Year holiday, Mr. Song was detained once again, because he gave the petitioners glutinous rice dumplings.
[...] After the coldest months of the winter had passed, I contacted Mr. Song and learned that he’d turned his focus toward rescuing petitioners who were being illegally detained in the infamous black jails, ad hoc detention centers that were set up in hotels to hold “troublemakers” from outside of Beijing until they could be returned forcibly to their hometowns.
[...] After the escape of the blind, barefoot lawyer Chen Guangcheng from his farmhouse in Shandong Province, where he’d been under illegal house arrest, Mr. Song took an even more dangerous risk. He drove to Dongshigu, Mr. Chen’s village, and helped the wife of Mr. Chen’s nephew, who had also been arrested, to escape to Beijing, where she went into hiding to avoid being abused by the local government.
Song's lawyer Liang Xiaojun gives a detailed account of their meeting in a detention center before Song's disapperance. From Yaxue Cao at Seeing Red in China:
I asked how he had been taken to custody and what the interrogation had been like. He spoke fast and clear: He was seized by policemen in the morning of May 4th while waiting in Beijing South Railway Station for a petitioner who had called and asked for his help in what now looked like a premeditated trap. He was then interrogated by policemen from Fengtai District Public Security Bureau and Beijing Headquarters respectively from the afternoon to early next morning. And as Xu Zhiyong predicted, it was about his visit to the black jail in Beijing set up by Chenzhou municipality, Hunan (湖南郴州) and his rescue of petitioners there, but also his online posts to help the petitioners. He was also asked his relationship with Xu Zhiyong—how he met him and how he became a volunteer for Citizen. On May 5, he was charged with “provoking disturbances” (寻衅滋事罪) and transferred to the Fengtai detention center.
[...] After that I was taken up by other obligations. I felt that Song Ze would be released soon, because, legally I couldn’t think of anything that he could possibly be convicted with. His detention was based on charges of “provoking disturbances” (寻衅滋事) as defined by Article 293 of China’s Criminal Law. They refer to the followings: beating another person at will; chasing, intercepting or hurling insults to another person; forcibly taking or demanding, willfully damaging, destroying or occupying public or private property; creating disturbances in a public place. As far as I could see, Song Ze had simply done what a citizen should have done, and he displayed no behaviors punishable by law.
Looking back now, I was too optimistic.
[...] On June 12 I went to Fengtai District detention center again. The officer in charge of the case told me that Song Ze had been switched to residing under surveillance and taken away by people from Beijing PSB a few days ago. He said he didn’t know which department of the PSB they were from, nor did he know where they had taken Song Ze. All he could tell me was that Fengtai District was no long on the case anymore.
Xu Zhiyong also expresses his concern about Song Ze's plight and explains the operation of black jails and surveillance in China. Translation by Hannah at Seeing Red in China:
Black prisons are places where local governments illegally detain petitioners. If the petitioners try to go to the Prime Minister’s house or foreign embassies near Dongjiaominxiang (东交民巷), Wangfujing Street (王府井大街) or other places where they are not supposed to petition, they could be taken away by police. During the so-called sensitive time of Two Meetings each year, they could be apprehended just passing through Chang’an Street (长安街) and being found carrying petitioning materials. All these are labeled “irregular petitioning” and the petitioners who have been rounded up are sent to Jiu Jing Zhuang (久敬庄), the detention and deportation center run by the State Bureau of Letters and Calls. Jiu Jing Zhuang would order local governments’ Beijing offices to take away petitioners from their jurisdictions on the same day they arrive in Jiu Jing Zhuang. However, most petitioners cannot be dispatched back to their homes that same day. They must wait to be sent home, perhaps needing a few days or a few weeks, and this turns into a profiteering opportunity for some people.
People running the black prisons are those who have connections with officials in the State Bureau of Letters and Calls or local governments’ Beijing offices. They rent hotel basements, hire thugs, forcibly take the petitioners from Jiu Jing Zhuang, illegally detain them, and then order the local governments to come to get the petitioners and pay a fee for the latters’ stay. They fetch 80 to 200 RMB per petitioner per day.
[...] In reality, residing under surveillance is more formidable than imprisonment. According to the new Criminal Procedure Law, the authority may designate the location for residing under surveillance, but it shall notify their relatives. But China being China, Song Ze’s family has not received any notification. He can still meet with his lawyer when detained in the detention center, but it’s been more than 40 days since he was put under residential surveillance, no one has been able to see Song Ze; and the PSB has refused to answer any questions on his whereabouts.