I don't think it's just the fact that he approached her, but that he circled the neighborhood several times scouting out a child, once he spotted her then he approached. The neighbors saw him circling. They must have been concerned to some extent to be watching diligently enough to see him grab her. If the entire ordeal only lasted 10 seconds, then something about him approaching her caught their attention as out of the ordinary to begin with.
I'm not faulting the witnesses, they did good chasing him after he abducted her, they got good information, etc.