Funnily enough the taxman is aware of these scams and, when the time comes for them to audit your employer (on average, every 6 years or so), they will run employees mileage claims through Autoroute and any mileage claimed above 105% or thereabouts of the software mileage for the journey will prompt questions.

No need to hit the panic button, just be aware of the risk, and try to avoid taking the piss because guess who is most likely to get their records checked?

And yes, my company had this happen just last year, so I am speaking from experience.