I lived in central Philly for a while. Being largely nocturnal, I'd often be out walking in the early morning. So one morning, I saw someone walking toward me,

I lived in central Philly for a while. Being largely nocturnal, I'd often be out walking in the early morning. So one morning, I saw someone walking toward me, wearing a stethoscope-like device, with a tube hanging down, almost touching the sidewalk. At first, I thought "crazy person, take care". But then, I saw a "Philadelphia Water Department" logo. Basically, he was listening for leaks. His partner was one block away, and they talked via radio. So they could triangulate.