Gerry McMahon makes his living as a brick mason, but on the side he’s become an authority on car break-ins. He’s learned all about one of San Francisco’s most rampant, frustrating crimes from a front-row seat — the house where he’s worked for the past several months on a stretch of Lombard Street where tourists park.

He looks for nondescript four-door vehicles, often silver or white, with blank paper license plates. Cars that never head down the famous curvy section of the street, but instead circle the blocks at the top of the hill — over and over again.

That’s how he knows yet another smash-and-grab specialist is on the prowl, searching for the visitor who has left a suitcase, backpack or pricey tech gadget in the car. That’s how he knows the crime wave tarnishing the reputation of San Francisco — where police received more than 30,000 reports of the crime last year, but made arrests in less than 2 percent of them — is about to get a bit worse.

As we talked the other day amid construction cones and loose bricks, I asked McMahon how many car break-ins he’s witnessed while on the job on Lombard.

“Easily 100, even more,” the 54-year-old Ireland native said in a thick brogue.

And how many arrests has he seen?

“Zero.”

Just then, we saw a white Volkswagen sedan with a paper license plate drive by and turn south onto Hyde Street. It circled and drove by again. And then again.

Scott Strazzante/The Chronicle

McMahon isn’t the only person on Lombard who has learned what trouble looks like. As the Volkswagen was circling, another man who noticed the car called police he knows at Central Station down the hill in North Beach, and in a few minutes several plainclothes officers, both on foot and in an unmarked car, began watching the VW.

Tina Bartlett Hinckley, a real estate broker and the owner of the Lombard Street home where McMahon has been working, had invited me out to see what she describes as “anarchy” at the famed tourist attraction. Every day, thieves break into cars on her block, with no consequence.

“There’s no risk,” she said. “Why would they stop?”

She and her neighbors do what they can. They call 911 frequently. They post handmade, laminated signs along the street warning tourists of the crimes. They sometimes let visitors with valuables in their cars park in their own garages.

McMahon has taken to telling tourists who park their cars that they should remove absolutely everything before proceeding to the twisty block. They usually wave him off and say they’ll be only a minute.

“Well, you’re giving ’em 55 seconds too long,” he tells them.

Here’s what can happen in those few seconds, McMahon says:

The car with no plates pulls up. Out jumps a passenger with a small tool — available online or in any hardware store — that’s intended to help people break windows if they’re stuck inside their own car.

You just press the tool against a window, and a spring-loaded spike shatters the glass. Because it doesn’t shake the car, alarms don’t go off. The thief grabs whatever is handy and returns to the getaway car.

Heat maps of where last year’s 30,000 police reports were taken in San Francisco show car break-ins are concentrated in tourist hot spots — from the Beach Chalet to Fisherman’s Wharf, from the Academy of Sciences to, yes, Lombard Street.

Sometimes, the losses are major: passports, cash, laptops, entire packed suitcases. Other times, they’re small yet devastating. McMahon vividly recalls the time parents with two little girls returned to their car to see the window smashed.

The older girl burst into tears. She’d just been released from UCSF Medical Center, where she was being treated for leukemia, and a teddy bear that she’d been given at the hospital was gone.

“She said, ‘Mom, take me to the airport right now — I want to leave,’” McMahon said.

Police came out to take a report that time, but said nothing could be done. That’s what they usually say, McMahon said.

“They shrugged. They see it all the time,” he said. “Some cops take it real personal. Others come, and they’re just going through the motions.”

As we talked, the Volkswagen drove by again and again, circling seven or eight times. Finally, it came to a stop next to a white Dodge Grand Caravan.

A man in a gray sweatshirt jumped out, quickly shattered the middle of three windows on the driver’s side, and reached in so far his waist was balanced on the door and his legs and feet were in the air. He emerged with a backpack and dashed back into the VW. It took just a few seconds.

The car sped off, again turning south on Hyde Street. This time it didn’t circle back around.

The undercover officers walking the area on foot weren’t close enough to do anything. Police in their car tried to follow the VW but lost it, and no arrests have been made, said Sgt. Steven Spagnuolo.

Smash-and-grab thieves would typically throw the backpack away and sell anything valuable at Seventh and Market streets, a popular place to unload stolen goods, he said.

After several minutes, a group of Chinese tourists returned to the Dodge. One woman who spoke a bit of English said they were friends visiting California for a week. They’d rented the van.

The backpack belonged to AnQi Li. Through a translator, he said it contained his passport, his Chinese identification card, 3,000 Hong Kong dollars (a little less than $400 in U.S. currency), a scarf, a selfie stick and a charger.

Scott Strazzante/The Chronicle

One of the undercover officers took a report. He asked Li if he’d be willing to return from China to testify, should police make an arrest. Under state law, a smashed window isn’t proof of a car break-in. The victim must testify in court that he or she had locked the car before leaving — it’s a loophole that state Sen. Scott Wiener is attempting to close with new legislation.

Pay to fly back from China to say his car was locked? The tourists laughed. The answer was no.

Asked whether the public perception that San Francisco police don’t really care about car break-ins is true, Spagnuolo said: “No. I do this every day. It’s our main focus.

“It kills us when people get their cars broken into,” he continued. “We’re the ones taking reports. We’re the ones talking to the kids that are crying. We want to catch these guys.”

His advice? Call 911 when you see anything suspicious, like a circling car with paper plates. Leave nothing in your car. If you drive a vehicle with no separate trunk, like a sport utility vehicle, use the cargo cover.

Mayor Mark Farrell told The Chronicle’s editorial board Wednesday that he’s directed his staff to conduct an analysis of arrests, bookings and convictions “to really get a holistic picture about what’s happening across our entire public safety system.”

He has also asked Police Chief Bill Scott to analyze how many police officers San Francisco really needs. The City Charter calls for 1,971, a figure Farrell said is an “artificial number” that might be too low.

In the meantime, I asked him if he’d direct city staffers to install signs at places frequented by tourists warning against leaving anything in their cars. There are a few such signs around Lombard, but only in English and not at the top of the street.

“Absolutely,” the mayor said.

Maybe such signs would have saved AnQi Li’s backpack. Maybe.

“You see it every day of the week,” said McMahon, the bricklayer. Soon, he knew, the Public Works crews would arrive.

“They’ll sweep up all the glass,” he said. “And then it will happen again.”

San Francisco Chronicle columnist Heather Knight appears Sundays and Tuesdays. Email: hknight@sfchronicle.com Twitter: @hknightsf

Gone in

39 seconds

A series of photos, time-stamped from when the white Volkswagen stopped, shows a man emerge from the car, peer into a tourist’s van, climb through a broken window and remove a backpack from inside — all in a matter of seconds.