What year is the car, what engine size ?
I assume it is quite new from that mileage, so will almost defo be discs on the rear, a flat disc that is easily seen and shiny.
It could be one of several things, the pads could be chomping thier way through rust on the disc that has formed around the inner edge, nearest the wheel bolts, this will make a noise, but will go and is not a concern.
cars that are driven gently do tend to rust this area of the disc badly some rust the whole width if used too gently, and this WILL lead to a grinding noise, not to be confused with a SQUEAL.
It can also be caused by seized rear calipers, but you certainly should have been advised of this if it is the case, as it is dangerous, but I don't believe from what you say it is the case.
Is the grinding getting less ?, fast fit outfits do not as said send pads back for money back, they are bought by the truck load, without warranty in some cases from the supplier.
Some cheap and nasty pads are just that and can cause all sort of noises.
If you were anywhere near myself I would gladly check change anything as necessary FOC for(you pay for parts).
tabetha