Stompah, the information contained in Carfax reports is neither exhaustive nor consistent.
That's why they use purposefully vague terminology such as "No Taxi Registration Reported", instead of "This vehicle has never been used as a taxi". It would be nearly impossible to collect the necessary data to provide a really reliable and useful report for every post-1981 U.S. automobile. And even if it were possible, it wouldn't be reasonable to expect such a report to contain data from other countries, for example, that a vehicle has served as a police cruiser in Canada or a taxi in Mexico prior to importation into the U.S.
Carfax relies upon systems such as Motor Vehicle Departments and Inspection programs all over the country to collect their data, and then to provide it to them in a cost-effective, timely manner. These entities, some run by the government, some under contract to private firms, sometimes just a chain of licensed repair shops, vary greatly in the quantity and quality of data that they provide.
Even when the system works well, there can be other factors which influence how much accurate information will be reflected in the report. The Connecticut Vehicle Inspection Program, normally a solid supplier of mileage numbers and inspection dates (very useful when looking for odometer fraud) was shut down early in 2002. It will be starting up again in a completely different form, but even if they immediately begin providing useful data, there will always be a gap of nearly 18 months in this particular bank of information.
Of course, we haven't discussed vehicles that aren't registered at all in the traditional sense. In many jurisdictions, Police, Fire Department, and other government-owned vehicles are not formally registered with their State's department of motor vehicles. Also, cars owned by auto dealers, repairers, transporters, and other similar enterprises are often "registered" by means of an interchangeable license plate, and no public record may exist anywhere of their use or transfer. It isn't reasonable to expect Carfax to have up-to-date information on such a vehicle.
It's best to think of a Carfax report as a tool, not a complete solution. Personally, I find that a quick chat with the last registered owner can be an excellent source of information. But sometimes, a carfax report is the only game in town. At least it is quick and easy, requiring only a VIN check. But it isn't perfect...that's why the much-touted "Carfax $5000 Guarantee" only pays you 10% of the car's value, not the full amount you paid for the car, if fraud is discovered.
FWIW, Carl