When protests swept Hong Kong last year, Alexandra Wong, better known as "Grandma Wong,” always seemed to be there. Day after day, she stood out among the hundreds of thousands of demonstrators as a small woman with short gray hair waving a large British flag.

Then, during a few tumultuous days in August when the police fired tear gas in a subway station and protesters shut down the city’s airport, Wong, 64, suddenly vanished.

Her friends in the movement quickly had their suspicions. They believed she had disappeared into China’s opaque legal system, an alarming outcome for protesters that has become all the more chilling after Beijing imposed a national security law on Hong Kong this year. When Wong finally reemerged last month, she confirmed that she had been detained by the Chinese authorities.