Inside a few clean-cut young men gathered, the room filled with guitars and drums. There were no pills, no bongs. For a few more days Liu stalked the group and soon his suspicions were confirmed. "Every day they didn't do anything other than band practice or going to the beach,'' Liu said. "Nothing bad."

As more mainland parents send their children to Hong Kong to study, a growing number of those who are wealthy - and suspicious - are paying detectives thousands of dollars to monitor their children. The targets range from kindergarten students to doctoral candidates, and share the background of being scions of prominent families travelling far from home to study.

Detectives say they employ a wealth of techniques - from planting cameras in water bottles to infiltrating students' social circles - to learn their targets' routines. While some parents fear that their children are experimenting with drugs or skipping classes, some detectives discover that their students' secret desires are as benign as becoming rock stars.

"Of course, you've got to suspect that something is seriously wrong with your kid before you'd come to a detective," said Mr So, manager of Asia International Investigation Bureau, who declined to reveal his full name.

Hong Kong parents have long used detectives to follow their children and ensure they're not getting into trouble. Mainland parents have caught on, and the clients are more willing to spend lots of money to know what's happening with their children, Liu said.

"What the mainland parents are worried about is not the money spent but the possibility that their children have fallen into the wrong crowd," said David Cheung, who has owned his own detective agency for three decades.

None of the detective agencies would permit the to speak with their clients, citing strict confidentiality agreements. It is difficult if not impossible to verify any of the stories they shared.

University administrators say they have received few cries for help from desperate parents.

Simon Lau, head of mainland and international student services at Polytechnic University, said that other than a parent orientation talk at the beginning of university life, PolyU offers "almost no further information for the parents".

But he said in his experience, he had never received any inquiry from mainland parents regarding the academic performance of their children, and just "a small number of phone calls and e-mails asking the university to help locate their children". All the students were found unharmed.

A few years ago, mainland clients started turning to Liu's agency to monitor cross-boundary kindergarten children who commute between Shenzhen and Hong Kong to attend lessons. "These kids, with no parents in Hong Kong, rely on nannies to bring them to school," Liu said. "So parents either ask us to monitor the nannies, or ask for devices to put in the kids' school bags to locate them." He said they took extra care not to disturb their young targets. "We would only film from afar or follow the nanny van in our own van."

Though such cases are on the rise, they are still small in number compared to cases involving university students, Liu said. "The parents just want to be reassured the kids are in safe hands," he said. The beginning of the school year is the peak season, bringing in a dozen cases, he said.

Inside the Tsim Sha Tsui offices of Global Investigation and Security Consultancy, owner Philic Man Hin-nam shows off a repertoire of gadgets that she uses to monitor her targets. They appear to be everyday items - pens, glasses, water bottles - but they all contain hidden cameras. Man says that her all-female agency has at least four experts dedicated to the research and development of such equipment.

If hi-tech surveillance is not enough, Man's company also provides undercover operations - meaning infiltrating the target's social circle by becoming his or her new friend. Man claims to have "all the usual expensive cars" for her undercover spies, and that her clients' children often have monthly allowances of more than HK$10,000.

"You cannot not drive a Ferrari when your target does, right?" she said.

Man declines to reveal how often mainland parents employ such extreme methods to spy on their children or how much it costs. She did, however, detail how such jobs are done. "We study each target's habits and tailor the surveillance device to them," she said. "A parent can give gifts to their children. Those could well be specially made gifts,'' meaning gifts that contain hidden cameras.

The detectives wouldn't discuss cases in which their targets realised they were being watched. Even if targets suspect, there's little they can do to stop the spying.

Michael Jackson, associate professor at the University of Hong Kong's Faculty of Law, said that Hong Kong had weak privacy protection, lagging behind many other jurisdictions. The Personal Data (Privacy) Ordinance, which is the closest that Hong Kong has to a privacy law, carries a general exemption for what it calls "domestic purposes", a category that allows parents to monitor their children.

Stalking is legal in Hong Kong, Jackson said. When children are being followed, photographed or secretly filmed in their rooms, some might argue that their privacy is being infringed. But they would have difficulty proving that the monitoring is unlawful.

"There is practically nothing to stop them from doing what they are doing," Jackson said, referring to both parents and the investigators they hire.

Man said that 90 per cent of the mainland family cases her company had taken on over seven years had found targets had drug or gambling problems. But most detectives said that most often parental fears were overblown.

Cheung said a mainland client once asked his firm to investigate a young woman who was a PhD candidate at Chinese University.

"The parents became alarmed when their daughter started spending large amounts of money," he said. "They feared she was being duped."

The agents sneaked onto the university campus and watched whether she was going to classes, where she went after class and who she was hanging out with.

The young lady was secretly following in her family's footsteps: she was investing in stocks, Cheung said. And doing quite well in that endeavour.