Sometimes when I open the door to get into my Model S, I hear these three "ding" sounds. I'm pretty sure that nothing has happened to my car while unattended. It happens in my garage at home occasionally, so the alarm hasn't gone off.
They are different from the alert noises made when both pedals are depressed. They happen before I am fully seated and before I have stepped on any pedals.
The video is from my dashcam. You'll hear the doors unlock, two doors open and then the "ding" sounds.
Model S door opening sounds - YouTube