Do you track mileage on your vehicles for maintenance purposes? Figure where you are sending them + or - for miscellaneous, and your mileage should be close.
Many of the laws have changed in the past few years on GPS and cell phone tracking and I would probably let all of my employees know that "for their safety" I was going to begin tracking company vehicles. No need to be specific about when or how, but putting them on notice that tracking was going to happen, may save you some headaches later on. It also might cause your questionable employees to consider their behavior.