It happens in plain sight under a freeway off ramp south of Market -- a man apparently disassembling bicycles that may or may not belong to him.
"They strip all the parts of it, put the frames in stacks, put the components in bins," bike theft victim Kevin Montgomery said.
Montgomery stumbled on the operation while looking for the bikes that were just stolen from him and his roommates.
"These are kind of part homeless encampments, part bicycle chop shops," San Francisco Police Ofc. Matt Friedman said.
Friedman is in charge of curbing bike theft. He's gotten to know a few of the guys on the street.
"They all have criminal records, they all claim that they're bike mechanics, they buy and sell parts, they buy and sell bike frames," he said.
It might sound shady, but that's not enough to make an arrest.
"A lot of that could be stolen," Friedman said. "I have ran countless serial numbers on these bicycles and they always come back, you know, unreported."
If you don't report your bike stolen, police will never catch the thieves.
"It's very important to have the serial number and the receipt," SAFE spokesperson Morgan St. Clair said.
Crime prevention group SAFE is going a step further. Next month, they'll launch a bike registration program.
"Our program will be run with the police department, so officers will be able to track bicycles," she said.
San Francisco police are setting out what they call "bait bikes," nice looking bikes with weak looking locks, and often one a camera nearby to record the theft as it happens.
"We just ran an operation yesterday where we have two felony arrests as a result of it," Friedman said. "You know we're using bikes with GPS-enabled trackers in them too, as well."
But the department has limited resources to attack a growing problem.
"In fact, the number of bikes stolen in San Francisco compared to iPhones is 3:1, so that gives you a sense of how serious bike theft is," San Francisco Bicycle Coalition spokesperson Kristen Smith said.
The bicycle coalition worries it'll keep people from riding. They're pushing for laws to let riders bring bikes inside at work. And like SAFE, they're spreading the word about how to lock your bike.
If you see a chop shop, call the police non-emergency line at 415-553-0123 and you can tweet pictures to @SFPDBikeTheft.