I think from experience of NHS disciplinaries, that a) if the company decide to put dashcams and driver cams in their vehicles, then they have the right to do so and the employee has the right to leave. They have a duty to ensure that the recordings are kept securely so that the employee’s data protection rights are not breached eg the recordings don’t end up on “Britain’s Most Stupid Drivers but a apart from that they have a right to monitor employees while they are at work.
b) If a breach of company T and C (or the law) happens then how that breach is discovered is not relevant. In a case I was involved in, evidence of continued fraudulent signing in and out was used as evidence to discipline an employee. The evidence was actually the fire safety record but it could still be used even though the offence was theft of company time and not related to safety.
But...I mean....come on....he knows there is a cam in the cab trained on him and he fails to use his seatbelt? as PP has kind of said....do you really want to ally yourself with an idiot?