My 2014 Mazda 3 6speed needs rear rotors and pads. Is there any history for thi

I have same situation . My 2014 Mazda3 has 24XXX miles on it and after 3 weeks of non use it started grinding noise on hard braking. Mechanic told me it was rust and it should go away after a day or two. It has been 4 days and I still hear grinding noise on hard braking. Mechanic says rotors and pads need to be replaced. He says I can keep driving like this for some more time. I think car has not many miles for rotor and brake pad replacement. Any suggestions and ideas are appreciated.