This couple that was in the same train as me, left their seats to change the diaper of their little baby.
Not only did they go away from their luggage, but they left two phones and a large tablet visible and unattended for quite some time!
That said, in rural Denmark, where I live, i have often forgotten my keys in my car and not even felt bad about it.
Actually my car door does not lock at all and hasn't done so for 3 years but leaving the key in the ignition could actually be illegal