While it’s highly unlikely that your car will roll away, it’s always a good idea to set your parking brake every time you’re… you know… parked. This is especially true if you’re on a ferry.
If you don’t, this is likely to happen;
Thankfully nobody was in the car at this time. This is one of the many reasons why you might not want to stay in your vehicle during a ferry ride. Be safe out there, folks, and remember to set your parking brakes!