If you wash your car ,even with brand new discs , you will get "corrosion" visible on the discs nearly immediately (
, this is one reason for pumping your brakes if youve been to a car wash,before you drive straight out on the road.
There will be a thin film of water still on the discs.
Use the brakes a couple of times and the instant rust will scrape straight off.
Pitting and probably grooves + possibly disc wear uneven may only give symptoms at low speed cos of uneven contact area.
Id look at the advisory as a bonus.I mean a free warning to change discs and obviously pads at the same time.
Unless the discs and pads have been changed recently and you would have expected them to last longer.. Id just stump up the cash.
Generic discs are as good as genuine oem ones in my experience.
In a previous life
I let the front discs get so worn they parted company with the wheel hub. 1 of them did I mean.. it actually looked like there was plenty of metal at the outer edge of the pad contact but the part in near the hub was paper thin and I never noticed.