Still, investigations generally fall into three categories－missing people, extramarital affairs and business disputes, says Meng Guanggang, who founded the country's first PI agency in 1993.

Meng points out affairs are increasingly common, especially among people in their 30s. "Some clients want to know who seduced their spouses, while others hope evidence can help them claim more property in the divorce," Wang Tong says.

Meng points out clients are "deeply influenced by gangster movies".

"They ask if we have the most advanced wiretaps, cameras and tracking tools," Meng says.

"They're disappointed to learn we use ordinary equipment. Unlike in the West, Chinese PIs can't touch criminal cases, which are the Public Security Ministry's exclusive domain. There's no need for advanced technology in civil cases. Sometimes a smartphone is enough. But IT is increasingly used in the Internet age."

Wang Tong's agency typically charges 5,000-7,000 yuan (about $800-1,150) for short and simple cases. The big bucks are in the business disputes. He made 70,000 yuan in the plastic film case, for instance.

Wang Dacheng declined to reveal his income but says it's "considerable".

"Case costs vary depending on difficulty and customers' ability to pay," he says. "Wealthy clients sometimes tip."

But satisfied customers don't translate into societal approval. Privacy infringement is one of the main concerns about the industry, Beijing Jihe Law Firm partner Wang Xiuquan explains.

Take extramarital affairs, for instance. Affairs violate the Marriage Law, but only evidence gathered in public can be used in court. Installing monitors in private property and hotel rooms are serious intrusions, he says. "It's a common practice in which private detectives track their targets and call the clients to deal with the situation when the man and mistress are inside a room," Meng says.