Liu Ping, a rights activist who has angered officials in China with her advocacy of free elections and support of labor and women’s rights issues, has been missing since early this month after she was detained in Beijing by security personnel from her hometown, according to an advocacy group. Wang Songlian, a researcher at Chinese Human Rights Defenders, said Monday that Liu’s disappearance was most likely part of a wave of detentions tied to the meetings of China’s handpicked Legislature, the National People’s Congress, and an advisory body, the Chinese People’s Political Consultative Conference.