Brake discs are bare metal and rust all the time. If you do not use your vehicle enough, especially during the winter when road salt is applied, surface corrosion will not be wiped off the discs and will take hold. It is very unusual for discs, even back discs that don't get used much in suburban braking, to rust to the point of needing replacement in such a low mileage, but there may be further reasons you have not appraised me of, such as using the X-Trail to launch boats down a slipway.