The first time I heard it was during a pitch black night in a mountain canyon of Nevada. I was sleeping in the bed of my truck (that had a shell). My female co-worker (we're field biologists) was sleeping on a cot about 50 yards away in a dry wash. After I heard the screams, I opened the shell back lid and called out in the darkness: Lisa... Is that you?
Silence, then "No!"
I grabbed my buck knife and went and got her. She set up her cot right next to my truck. She could have slept in the cab, but she insisted on staying outside. (Just so you know, sharing the bed of the truck with me was not an option. I'm happily married and wish to stay that way.)
Being at work alone in a guardshack and hearing that noise directly outside your window immediately behind you gets your attention right quick.