A couple lived near the ocean and used to walk the beach a lot.
One summer they noticed a girl who was at the beach pretty much every day. She wasn't unusual, nor was the travel bag she carried, except for one thing; she would approach people who were sitting on the beach, glance around furtively, then speak to them.
Generally, the people would respond negatively and she would wander off, but occasionally someone would nod and there would be a quick exchange of money for something she carried in her bag.
The couple assumed she was selling drugs and debated calling the cops, but since they didn't know for sure they just continued to watch her.