That was just one of a dozen big cases Wang and colleagues take a year. There are about 200,000 PIs in China, compared to about a dozen private detectives working for a single agency in the 1990s, insiders say. There are no official figures.

Wang entered the sector in 1993 and started his own agency a decade later. He points out detectives look different from film portrayals. "You've screwed up if people can tell you are a private detective," he says. "We try to be the most inconspicuous people in a crowd. Sometimes, we pretend to be delivery men or cleaners to gather intelligence."

His first case was to track a doctor. He failed. "People rode bicycles in those days," he recalls. "I followed him for more than 20 kilometers on a cold winter day. My hands were nearly frostbitten. Yet I got nothing useful."

Mu says most cases he takes are economic disputes but are still risky. "Simple business disputes can be complicated, especially when the investigation involves officials or executives," says the 50-year-old former policeman. "We often receive threatening messages and e-mails, and even get knives in the mail."

Mu terminated some cases because keeping a distance from politics is a rule of thumb.

Wang says the job entails "pressure".

"We see society's dark side every day," he says. "I bought a house on the coast so the sounds of the waves will soothe my soul."

Not only has the sector's size expanded since its inception but also has its scope. It deals with much more than suspicious spouses, fraud and corruption.

The industry has chiseled out China-specific contours as it has taken shape. Parents hire companies such as China Sai'an to monitor their children's school lives and performances, and to make sure they don't have secret puppy love or hang around the wrong crowd.

While spying on spouses isn't new, more Chinese are paying for "premarital personal character investigations" from such companies as Feidu Detectives before deciding to commit. Zhao Yan Investigation Services helps Chinese parents who fear their children may be spending too much time in Internet cafes and foreigners who are perhaps being scammed by sketchy local suitors.

"We can do anything you want," says 34-year-old Wang Dacheng, who runs an agency in Jilin province's capital Changchun.