Tax experts say rumours abound about inspectors out spotting for company vans - to nail private users at the shops.
A change in the law, which came in from mid-April, means employees are now allowed to drive the vehicles to and from work without being taxed on the benefit.
But if they use the vans outside of that, then a charge kicks in.
And Graham Appleby, tax consultant at Howarth Clark Whitehill in Kidderminster, said for the last few months he and colleagues had heard rumours that inspectors had been checking supermarket car parks on Saturdays.
The claim is that they peruse registration numbers and names of businesses on vans and later check to see if the vans are available for unrestricted private use.
Mr Appleby said employees who use a company van for private mileage have always been taxed on the benefit even if the private use was restricted to journeys between their home and place of work.
Where a van was made available for private use, the benefit charge was £500 or £350 for vans more than four years old at the end of the tax year.