I'm sure each of us has seen young girls who for a second you think look like Madeleine, but it's the Madeleine of eight years ago. That's a more likely reason why someone might not report it, they just know it's extremely unlikely.
Some people clearly do still report these sightings though, as a newspaper report in this thread indicates.
But I wasn't thinking about sightings, but about someone stumbling across non-visual evidence - hearing something or seeing other evidence. In those cases we'd all report it because you wouldn't know when you stumbled upon evidence of a child being held or mistreated who the child is, and so the rational side of your brain wouldn't intervene to tell you to get a grip.
I would hope so. However, until around 10 years or so ago, I'm not sure that I would have taken much notice of odd details that I might have stumbled upon in the course of a normal, busy day.
A case in point is an amazing story about a guy in Germany who was sorting out his household rubbish at a recycling site. He came across a scrap of paper in the waste-paper bin area with a help message scribbled on it. Instead of ignoring it, or just picking it up and dumping it with the rest of the waste paper, he called the police.
It turned out to be a desperate plea from a kidnapped child, who'd finally won the trust of her captor, and who could accompany him to the recycling point. She'd scribbled who she was and where she was on that bit of paper and had managed to drop it without her captor realising.
