As Li’s teammates gather around him, they take pictures from every possible angle so they can report the transgression to the bike-sharing company. “This is awful — how dare they?” whispers one. Another congratulates Li: “You are such a personal lock bloodhound!”

The gathering causes a small scene in the alley. People nearby stop what they are doing, curious to see what’s going on. Zeng lowers his voice and says, “Let’s keep it quiet here.” If the bike user were to discover them and get angry, it wouldn’t be the first time. “It’s dangerous for us,” Zeng says. “The user might still be around.”

In less than a year, shared bicycles have taken over several Chinese cities. The companies behind the boom — Mobike, Ofo, and a steadily increasing number of competitors — are popular among consumers and have become investor darlings. Their colorful bikes are now so ubiquitous that several municipalities are considering new legislation to keep their crowded sidewalks somewhat usable.

The bikes are commonly equipped with GPS and can be left anywhere in public. Unlike previous shared-bicycle projects, there are no “stations” to which they must be returned. The setup is convenient, but also exploitable. What if users take the bikes home or use a lock of their own to make sure other people won’t be able to ride them?

That’s where the bike hunters come in. There are several steps to their game: scouting locations using the companies’ smartphone apps, exploring the area to find the bikes, collecting photographic evidence, and reporting the infractions.

The bike-sharing companies will know who last used the bike and add a mark to the user’s record. Mobike, for example, uses a point system: Users start out with 100, and 20 points are deducted for each infraction. Rates go up significantly if a user’s point total drops below 80. As for the hunters, they gain one point each time they successfully report a case.

The bike-hunting game, with whiffs of danger and mystery, was invented by Zhuang Ji, marketing manager at Power Station of Art, a contemporary art museum in Shanghai.

The museum is located in a quiet corner of the city, near the Huangpu River; there are no subway stations within walking distance, and only one bus line passes through the neighborhood. Ever since 42-year-old Zhuang started working at the museum three years ago, he has been pondering how to make it easier to reach. “It’s a dead end here,” he says. “I’ve tried so many ways to solve the problem of the last 1.5 kilometers. Everything failed until May 6, 2016, when Mobike launched.”

Zhuang contacted the company right away, and they agreed to drop off 60 bikes at the closest subway stop. “I was so happy to see this result,” Zhuang recalls. “However, after the first day, the bikes started to go missing.” Angry that his long-sought solution was being ruined, he went out to look for the missing bikes after work.

Since that first hunt, hundreds of bike-sharing enthusiasts from all over China have followed Zhuang’s lead. “Once you play the game, you get addicted,” he says. “It’s even better than ‘Pokémon Go’ because it’s real.”

Li, the personal-lock bloodhound, says every hunt feels “absolutely amazing.” The 15-year-old gets a rush from finding infractions. “A personal lock is kind of like the big boss of the game,” he says, adding that he can’t understand why people would do such a thing. “When someone uses a personal lock on a public bike, that’s stealing.”

The bike hunters share tips and war stories on massaging app WeChat. They find bikes in the strangest places: hanging from trees, parked inside office buildings, abandoned on overpasses, and sometimes even thrown into bodies of water.

Zeng is the subject of one of the community’s most legendary stories. The 30-year-old IT worker once confronted a gang of seven people at the scene of an infraction — or at least that’s one version of the story. Zeng laughs at the embellished retellings and says the incident was “no big deal.”

It happened one evening while Zeng was solo hunting in his neighborhood: “I enjoy hunting in the shadows,” he says. He had found an improperly parked bike, and, while he was gathering evidence, a man yelled at him: “What the fuck are you doing? Mind your own business!” The man and two others approached Zeng, and an argument ensued. But when the aggressors spotted a security camera, they left. “A hunter has to keep a low profile,” Zeng says. “No one can protect us when things like this happen.”

With all their hunting experience, Zeng and Li tried to enlist the help of authorities in personal lock cases. But to their disappointment, the local police refused to help them. “We can’t do it by ourselves,” says Zeng. “The government should take action and make things right.”

But other governments seem more willing to cooperate. Shenzhen, a city close to Guangzhou and also in Guangdong province, recently introduced draft regulations for shared bikes stipulating that authorities may adjust the credit scores of users caught breaking the rules, such as parking in non-designated spaces.

Zhuang thinks it’s the users’ responsibility to follow the rules. “I hope our presence can change some of the bad behavior from cyclists,” he says. An optimist, Zeng doesn’t think bike hunters will be necessary in the future: Once the companies have enough bicycles, he believes, there will be less of an incentive for users to skirt terms of use.

But regardless of whether they’re needed, the hunters will still be around. “They don’t just show up because there are shared bikes,” Zhuang says. “They exist in society. If they weren’t hunting bikes, they’d be assisting at the scene of an earthquake, or anywhere help is needed.”