One morning last August, a 15-year-old autistic boy called Yu Man-hon ran across the border. He was returned to Hong Kong, where immigration officials decided he must have come from the mainland - part of their reasoning being that his clothing seemed too cheap for a Hong Kong youth - and sent him back to Shenzhen. He has not been found since. You know this story of course, because it has been in the newspapers; the frequency of its appearance has dwindled, however, because what more is there to say? We have all turned the page to read of other outrages. Yu Lai Wai-ling is Man-hon's mother. Every morning at 7.30 she leaves the family flat in Lok Fu, crosses the border, sits in a hotel room in Shenzhen and waits to hear news about her son. Every evening, she comes back to Hong Kong. In the early days of Man-hon's disappearance she stayed in Shenzhen, but she has to take care of his 14-year-old brother, Yu Yu-cho, and her husband, Yu Pui-lam, a caretaker on a Kwun Tong estate. Pui-lam, the breadwinner, is on anti-depressants. So now she takes the route Man-hon did, and the closer she gets to Lok Fu on her way back every night, the further she feels from her lost son. I went to see her with my colleague, Vince Lung. It was a dazzling, light-filled December afternoon and the 20th-floor family flat was like a suntrap. I wrote that word unthinkingly, but it occurs to me it is apt because Man-hon was always trying to escape. A key hangs on a long ribbon near the kitchen. When Man-hon and his mother came home from school every day, she locked the front door immediately and hung the key round her neck. She made sure the windows were fastened. She had a nightmare that Man-hon would run away and she would never find him. Man-hon, born in 1985, was a cheerful child. Yu, 47, is a pale, fragile woman with a limp - the result of a muscle-wasting illness in her childhood - and was told by a doctor that Man-hon was autistic when he was two. She used to carry both her children, and their milk bottles and congee, round with her 'like a refugee' when she went to see specialists. In those days the stress was physical. Later, as Man-hon grew bigger and faster on his feet, the burden became mental. I thought last August's was Man-hon's first escape, but he went missing for seven days in 1998. He came home from school as usual with his mother, and as she put down her bag and reached for the key he slipped out and ran. That night, he was found hiding in a private housing estate in Ma On Shan; the residents thought he was an illegal immigrant and he was taken to the Prince of Wales hospital in Sha Tin, but it was a week before the police got round to telling his mother where he was. I asked why it took so long, and Yu said she'd never had an explanation. At that moment I realised why she keeps believing, almost four months later, that her son will be found. She marked his clothes with his name and address and gave him a wrist-tag, but Man-hon hated the feeling of being confined so he threw those labels away. After that, Yu, who used to work nights at a 7-Eleven, gave up work and began treatment for stress. At the beginning of summer Man-hon bolted in Lok Fu MTR station. The police found him, hours later, sitting in McDonald's in Diamond Hill. When he ran away from Yau Ma Tei station, in the early afternoon of Thursday, August 24, he headed north. The first clue Yu had that he might be on the mainland came when a friend told her she'd glimpsed him on the Chinese side of the border the following day. His mother believes he knew it was the way home but he couldn't dodge the police, as he had the previous day, and was trapped. He hung around the railway station for a few days, where someone saw him being beaten with a broom while he begged at a noodle shop. Then the reliable sightings ended, and the crank calls - cruel, greedy, unforgiveable, prompted by an initial, anonymously offered reward of $1 million for information on his whereabouts - began. Yu did not cry while she talked about these things, I think because she has become used to strangers asking her about Man-hon. In a curious way, she has already known what it is like to be an object of curiosity: she said people always glared at her when Man-hon yelled and gesticulated on the street, and that now, at least, the public in Hong Kong is kinder in its recognition of her. It took his disappearance to make such small humanity possible. Hardship did not arrive suddenly in her life on August 24; she recognised its taste long before then. Every day in Shenzhen she is accompanied by two reporters from Apple Daily, two from the Oriental Daily News and two from The Sun. I winced at that but she said no, she was glad to have the company, especially when the conmen called, because it made her feel safer. I wanted to know - hating the question and myself because of what it implied - how long she would keep going up there, and she said she would continue until she found Man-hon, or his body. She needs to grasp what happened; she does not want him to vanish, inexplicably, forever. I asked to see Man-hon's room and we gazed at his bed for a while in the sun. His mother still sleeps in the bed next to it, while his father sleeps in the sitting-room and Yu-cho in the other bedroom. A few weeks after Man-hon's disappearance, the Yu family consulted a feng shui expert who recommended a golden-coloured quilt and pillowcase and a tank filled with bright, endlessly pulsating plastic fish. There is a lai-see packet under the pillow. Nothing is to be moved until Man-hon returns. A huge poster of the Hong Kong cityscape, pictured from The Peak, hangs above the empty bed. Yu explained with a smile that it was there to hide the scribbles on the wall behind, but it made me think how something in Man-hon yearned to run into those streets. On that August morning, the family had dim sum in Yau Ma Tei and his mother says Man-hon ate plenty and was happy. Ahead of him there were handcuffs and unkindness and awful fear, but at that moment when he sprang out of the train carriage at Yau Ma Tei and began his journey into the headlines, he thought he was free.