An exciting electric current of discovery runs through “Finding Vivian Maier,” a documentary about a street photographer who never exhibited her work. She scarcely shared it even with those who knew her. Then again, many of her acquaintances when she was taking some of her remarkable images, particularly in and around Chicago in the 1950s and ’60s, were the children she cared for while working as a nanny. Later in her life, some of those children took care of her in turn, first by moving her into an apartment and then the nursing home where she died in 2009. What rotten timing: She was on the verge of being discovered, first as a curiosity and then as a social-media sensation and a mystery.

It’s no surprise that Maier is now the subject of a documentary, given the quality of her work, the nominal exoticism of her life and the secrets that still drift around her. She’s a terrific story — part Mary Poppins, part Weegee — who was at once emancipated and in service. She was introduced to the world, as it were, by John Maloof, one of this movie’s directors, who bought a box of her negatives at a Chicago auction in 2007 for about $400. The auction house, he explains, told him the work was by Maier, but he found nothing about her on Google. He had purchased the negatives for a book he was working on, but after deciding that they were of no use, he stashed the box away.

And then he took it out again, scanned some images and put them up on Flickr. “I have a ton of her work (about 30-40,000 negatives), which ranges in dates from the 1950s-1970s,” he wrote there in October 2009. “Is this type of work worthy of exhibitions, a book? Or do bodies of work like this come up often? Any direction would be great.”