That happened to me once the first week I got it.
Went to Lowes and the car turned off! someone I locked, it beeped and my family and I went inside! It took about an hour and half, when we went outside and opened the trunk I didn't head the car on, but I saw the Navi on from behind, we went in and the car was freaking cold! LOL, but yes, I was very upset and I don't know how the car turned on by itself...
That was really weird.. anyone could have taken it for a spin... oh and yes, the car was unlocked!!!