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!
Some parts of the US are like this, usually affluent cities where strangers are willing to yell at other strangers on your behalf / out of a sense of collective justice. But they were probably paying attention to your stuff already because they thought it was suspicious that someone got up and left behind a whole-ass bag.