Meng Guanggang in his early years. Photo provided to China Daily

Meng knows the legal boundaries of the profession since he's a former police officer.

Wang Tong, who started an agency in Liaoning's Dalian city, says Chinese PIs don't have investigative legal powers like their Western peers. "So it's understandable we've used illicit methods," he says.

He points to privacy invasion. "China didn't attach much importance to privacy rights in the '90s," he says. "Work was easier then."

Meng has organized industry conferences with law professors and experts in hopes the industry can develop through self-regulation. But authorities have resisted, he says.

He has given up hope the sector will gain a legal status after more than three decades. So, he's leaving.

As is Beijing detective Mu Yeyue. "I like my profession but must exit," he says.

He plans to found an IT company with friends. Meng doesn't take cases aside from some recommended by friends, and he is trying his hand at writing. "What I've done over the past decades is help people find justice," he says.