Car door automatically locks when I'm outside of the car?

[wdkwang]

I already understand how the car automatically locks once I'm driving past 15mph and it automatically unlocks once I turn off the ignition.
Also, I know that if I unlock the doors from the outside and don't actually open the door, it will automatically lock itself again in a minute's time.

What I wanted to ask was what is the specific situation and/or time that would cause the doors to lock itself after I shut off the car, exit and close the door, but walk away without locking the doors.
What caused me to think about this was how my girlfriend told me the car locked itself eventually when she sat in the car waiting while I went across the street to get something from the store.
I found this strange because it never automatically locked while I was washing the car, or going in and out of the house for tools while working on it.
Does anyone have a clarification for this?