San Francisco’s epidemic of car burglaries may be spreading even faster than the already alarming 28 percent increase reported by police this year.

Statistics obtained from the city’s 911 center show it received 25,031 calls about auto break-ins during the first six months of 2017 — 7,061 more than the 17,970 reported by police.

The difference is that car-burglary victims’ first reaction is often to call 911 — but they don’t always follow through by filing an online report, and the cops don’t send anyone to the scene unless a smash-and-grab is in progress.

What goes into the official crime stats? Police responses and victims’ reports. A 911 call with no follow-up doesn’t count.

Police spokesman Michael Andraychak cautioned that 911 calls could include reports from several witnesses of a single break-in. But there’s no doubt some burglaries simply aren’t registering in the stats.

Supervisor Hillary Ronen, whose district includes the Mission, ground zero for the break-in explosion, said she “wouldn’t be surprised if the number (for the year to date) is even higher than 25,000.”

Ronen said she’s talked to several people who have stopped bothering to file police reports.

“I’ve had people tell me this being their third or fourth break-in,” Ronen said. “It’s like a tax for living here.”

In fact, after five break-ins, Ronen said, “one woman in Bernal Heights put a sign out saying, ‘F— you. The car is unlocked. There is nothing in the car. Nothing in the glove box.’”

Tony Ribera, a former San Francisco police chief who now teaches criminal justice at the University of San Francisco, said cops tell him auto break-ins are often done by petty criminals working in teams. With auto burglary classified as a nonviolent crime, and citations issued for possession of stolen goods worth less than $950, those arrested often roll in and out of the court system.

Police reported making 303 arrests in the first six months of the year.

“Some of them have been arrested 15 to 20 times,” Ribera said. “I mean, it is ridiculous.”

Tower talk: Millennium Tower condo owners say they’ve got proof that planners of the next-door Transbay Transit Center ignored their legal obligation to prevent the residential high-rise from sinking.

Lawyers for 250 Millennium homeowners who are suing the Transbay Joint Powers Authority, the public agency building the mega bus station, have filed a freshly amended complaint pointing to the transit center’s pre-construction environmental impact report. The document said the authority was required to do prep work to “protect existing structures from excessive ground movements during construction” and “monitor adjacent buildings for movement.”

If any buildings were indeed moving — like, sinking into the ground or leaning to one side — the transit center’s builders would “take immediate action to control the movement,” the report said.

Despite the monitoring, none of the other steps happened, and the consequences were disastrous, said David Casselman, lead attorney for the homeowners.

In fact, the Millennium had already sunk 10 inches by time the digging began for the $2.4 billion bus and rail hub in 2010. Casselman said “the bulk of that was expected building settlement” and it had all but stopped before transit center construction began.

As of July, the skyscraper had sunk another 7 inches and was tilting nearly 14 inches to the northwest at the roofline, increasing “the likelihood that it will become impossible to return the building” to its original upright position, the lawsuit says.

Transbay authority officials are sticking to their defense that the Millennium was sunk by a bad design, one that didn’t provide for pushing the 58-story building’s pilings to bedrock.

“Taxpayers should not be on the hook for the developer’s shortcuts,” the authority’s lawyers said in a statement.

“The Millennium Tower had settled twice as much as the developer and its geotechnical consultant had predicted before (the authority) began its excavation ... and is continuing to sink and tilt at about the same rate even after (the authority) stopped digging,” the lawyers said.

There are practical reasons that the homeowners are suing the public authority and not Millennium Partners, the building’s developer: If they win, they keep the entire judgment — with their attorney and expert witness fees charged separately to the transbay agency, under laws designed to protect private property owners from public excesses.

Not to mention that the transbay authority would have an easier time paying up — because, unlike the developer, it’s a public entity and can float a bond to pay for the judgment.

San Francisco Chronicle columnists Phillip Matier and Andrew Ross appear Sundays, Mondays and Wednesdays. Matier can be seen on the KPIX TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call (415) 777-8815, or email matierandross@sfchronicle.com. Twitter: @matierandross