In 2007, when 26-year-old Chicago real estate agent, amateur historian and storage-auction enthusiast, John Maloof was on the hunt for images to illustrate a history book, he never thought he would stumble upon the work of an artist now being hailed as one of America’s most prolific 20th-century street photographers.

The story is like an Antiques Roadshow-esque urban myth. Maloof bid $380 on a box of old negatives at an auction house, hoping to find some useful images of old-world Chicago. As it turns out, the images weren’t useful for that project. But when he returned to the mysterious box a few months later he discovered breathtaking shots of glamorous creatures dressed in Dior’s “New Look”, children playing in the street and the expressive faces of Chicago’s and New York’s toughest streets. These were the (unpublished) works of Vivian Maier, a nanny and amateur photographer who had recently passed away.

Enthralled by the mystery of this photographer, Maloof hunted down the rest of the boxes sold at auction, and ended up with close to 150,000 negatives, 3000 prints, hundreds of rolls of film, movies and audio recordings – all never seen before. When he began to dig into exactly who Vivian Maier was, he unravelled a compelling story of an eccentric loner and a skilled voyeur, fascinated by the human condition and with an eye akin to masters such as Diane Arbus, Robert Frank and Helen Levitt.

Finding Vivian Maier is the 2013 documentary that explores the woman behind the work Maloof uncovered. As he interviews Maier’s ex-employers and locals who remember her, it becomes clear just how mysterious and private she was. “She was a very solitary person. She would never have let this happen,” says one interviewee of the unearthing of her work. “She would have seen this as an intrusion,” says another. The children she cared for recall the permanent fixture around her neck: a simple Rolleiflex camera. They talk of daily trips to the city, the slums, department stores and the backstreets, where she would watch, snap and stalk her subjects, all completely unaware of the talent behind the lens.

Maier’s absolute desire for privacy is reiterated over and over again by those who knew her best. And this brings a moral ambiguity to the film. One of the local shopkeepers says,“I find the mystery of it more interesting than the work itself. I’d love to know more about this person and I don’t think you can do that through her work.”

It’s one thing to hang Maier’s brilliant oeuvre up in a gallery. But to delve into her private thoughts, movements, belongings and desires, and to put them on the big screen might be going too far. But, like street photography, documentary filmmaking is concerned with capturing the true stories of a time and place. Whether she would have approved or not, Maier’s legacy is worth celebrating.

Finding Vivian Maier is showing nationally from October 31.

findingvivianmaier.com