The scene of protagonist Archie Bollen and his girlfriend May sitting on the balcony of a charming Victorian house in San Francisco lasts only a few seconds in the 1968 psychedelic movie Petulia. But it sent Brian Hollins, who’s adopted the nickname City Sleuth, on a weeks-long hunt to track down its exact location in real life.

Hollins is a theatrical detective, a retired Silicon Valley guy who runs Reel SF, a blog dedicated to pinpointing San Francisco locations depicted in classic films, many of which are film noir. The Victorian house proved to be a challenge. It’s striking: a vibrant, blue two-story home with ornate white columns that guide the eyes to a lavish front porch and that give the building a sort of elegant-gingerbread-house feel. But there are virtually no clues that reveal its location: no street sign, no distinct buildings nearby, not even a shot of the city skyline.

Searches on Google turned up nothing, and visits to blogs and IMDb proved fruitless. “I couldn't find it anywhere,” Hollins says. But then, an unexpected breakthrough.

“One day, I was driving back from the opera with my wife and I made a right turn on a street—the same right turn I've made for the last 10 years going home from the opera,” he says. “I just happened to look at the house on the corner and I recognized it instantly.” That’s it!

It turned out to be the Talbot-Dutton House, located on the corner of Pacific Avenue and Franklin Street. Built by the lumber businessman William Talbot in the 1860s as a wedding gift to his daughter, it’s preserved today as a historic landmark. As Hollins recalls, the house looked exactly the same in 2011 as it did in the film Petulia. The house is one of the 27 locations he’s tracked down so far for that movie alone.

Five years later, Hollins still hasn’t shaken off the excitement. He dubs triumphs like this one his “eureka moments,” and says it’s what kept up his passion for the project over the last 16 years (and counting). In that time, he’s sought out every location he possibly could for nearly 30 classic movies. He’s found the rooftop from which detective John Ferguson watches his colleague fall to hisdeath in the dramatic opening of Alfred Hitchcock’s 1958 Vertigo. He’s traced every location of the beloved 13-minute car chase in the 1968 thriller Bullitt.Hollins even found the exact spot the crew filmed the opening scene to the 1949 movie Thieves Highway using a peculiarly shaped building captured way in the distance as his sole clue.

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On his blog, Hollins posts his new shots with the original scene to show the “then and now” changes. Some sites have either become unrecognizable or vanished altogether; in other neighborhoods, remnants of the past—an old hotel, for example—still stand. ”You’re looking across the financial district, say, at the bay, and you compare it with today,” Hollins says, “and my gosh, it's just different with all the new buildings and high-rises."