Electromechanical parking brakes usually (not always) auto-release when you set off. There are two types: Single switch where the parking brake has to be switched on, but normally auto releases when you set off. Or double switch where the parking brake switch has to be switched on and has to be switched off before the vehicle can be moved. The second 'brake hold' switch has to be switched on once the car is moving and then holds the car on the brakes whenever you brake to a complete stop, automatically releasing the brakes when you set off again. Your car was fitted with the former type and you can contend that you should have been instructed how the brake functioned before you drove off and by not instructing you the car rental company was negligent.