The bright orange sack is designed to remind drivers to remove valuables from their vehicle. Drivers place valuables in the sack, which is then placed inside a briefcase or a handbag and taken out of the car.
A windscreen sticker says the car has been emptied of all valuables.
The bag contains a leaflet of security tips to help drivers combat vehicle crime. The 'safe sack' is available free at any RAC Auto Windscreens site.
Richard Usher, RAC Auto Windscreens marketing and communications director, said that many fleet drivers had the attitude that their vehicle, and much of its contents, did not belong to them and so the risk to their own pocket was negligible.
'Fleet car drivers who fall into this trap are setting themselves up for a rude awakening,' said Usher.
'Many companies do not offer contents insurance on their company car or if they do the excess is so high as to be virtually worthless, making the driver personally liable for the replacement of any goods lost.
'In the case of a sales rep, who may have a number of expensive samples as well as a laptop, the cost could run into thousands of pounds.'