Be very sure on the specifics of this.
Bags can and often do travel without the accompanying passenger, as knobbygb says usually when they've been left behind. They are then known as "Rush" bugs and undergo extensive screening to ensure the contents are acceptable and are marked as such. How else could your bag be returned to you should it go missing.
If a bag has remained onboard when a passenger has no showed then it is a very different matter, and there will be consequences. Even if the dispatcher at origin does not spot the bag count discrepancy, it will be spotted on arrival and there are severe penalties.
Are you 100% certain it was not a rush bag?