sidewalk hero

A person that looks/acts like a civilian in public but will jump into action when a medical or social crisis hits outside of typical occupations/environments, typically on public sidewalks.
I.E. people that treat injured people or stop criminals before they escape the scene of the crime

"Did you hear about that woman that saved a girl's life in the news? Patched her injury up to keep her alive until the paramedics arrived!"
"Yeah, she was a real sidewalk hero!"