My dad and I were on a walk down a typically nice, city sidewalk that bordered a Cherry Creek in Denver (a fairly populated place). We ended up walking through a section that went under a road and smelled something absolutely foul waft through the breeze from the tall weeds/grass that was growing by the creek.
I told my dad that was the smell of decay. Even as a naive teenager, I knew that smell. He didn't believe me, stating that it was just the smell of someone who had used the creek as a bathroom (admittedly a common occurrence in that area). I thought he was crazy because it was absolutely unmistakable to me. I had the inkling that we should've stopped and called the non-emergency line for the police, but he talked me into thinking it was nothing and we moved on.
Two days later, they found the body of a 20-something year old guy. It had been ditched in that exact spot after he was murdered. I do occasionally wonder if the investigation would've gone more smoothly if we would've called the police or something before the body decomposed even further than it had when they found it.