It's called Seeking Mavis Beacon (and that website is pretty great). Sadly predictable: "The model for Mavis is a very real Haitian immigrant named Renée L’Espérance, who was paid $500 for her likeness before falling off the grid."
Here's another review, from Engadget, that seems to embrace the film's vibe better than the Slant one I linked above. The articles claim the filmmakers are big on making themselves characters, and that seems to be true, but it also seems to be true of most of these documentaries following filmmakers as they unravel a real-life mystery, and few people had a problem when, say, the Toynbee tiles documentary guy did it.
