Carfax, carcheck, etc, work off the same national insurance and DMV/BMV database. Carfax only can report what is reported to them. There can even be reports on the national database that are not reported to Carfax. So, a car can absolutely have had minor damage all the way to being totaled without being reported to Carfax. A vehicle can be repaired by several ways that would not be reported. Privately, independent bodyshops, dealerships not making an insurance claim and other ways that would never be reported to Carfax.
I, myself bought a late model Mercedes that had a squeaky clean Carfax but yet had in the recent past significant body damage to the rear. It had been repaired by a dealership but was paid privately by the owner and no insurance claim or DMV report was made. Only after I had it extensively checked out did I discover the damage. It took quite a bit of digging because the repairing dealership had done an excellent job in the repair. I bought the car anyway but the Mercedes dealership had to come way down on the price after initially playing hardball, claiming via Carfax that it was damage free.
Even Carfax's buyback guarantee only applies if you can show they did not report something that had been reported to them. Many people think their buyback guarantee means if their report shows clean, and it turns out there is damage, they have to buy the car back. That is not the case. Carfax is a tool. It is a reference point and I would recommend it as part and only part of checking a vehicle out. Carfax and the like are hardly absolute references and it would be a mistake to depend on them as such.
