Manufacturer 'certified' used cars typically are limited to 3-4 years old and <50,000 miles, though that varies by manufacturer.
It may have been in a minor accident, though that is highly unlikely. The manufacturers typically try to keep strict quality control over their certified cars, because they want people to trust the idea of certification. They will only choose their best trade-ins and off-leases to certify and resell at a premium over non-certified cars.
It will not have been salvaged under any circumstances, because all cars that are totalled by insurance (even by flood or fire) are required to carry a permanent 'SALVAGE' title indicating this. No American insurance company will give more than liability coverage to a car with a salvage title... so no company-affiliated dealer will sell one.
Carfax will definitely flag a salvage title or even most accident reports. I think it's $25 for a 30-day unlimited subscription, meaning you can research the VIN of as many cars as you want in that time frame. Well worth it if you're shopping used.