May I ask what's the place where you heard this?
Also, if it is really haunting you, you can make an anonymous call to the police reporting what you experienced. Since I don't know if where you heard it was close to some abandoned building, woods, etc, I can't really help but think that it may have been some sort of crime. Maybe they will find something.
On the other hand, if it was in the middle of nowhere, it is indeed pretty strange. Do you usually drive that route, or was this a special case?