i did new rotors and pads last year however i still do notice the occasional rear 'overnight rust' noise on them which i thought would go away with new hardware.
I'm wondering if you are actually hearing the ABS fire, or more likely the traction control (applying the brake).
My 2013 was applying the right front brake, usually when I was pulling out of the driveway or a parking space, and had cut the wheel all the way. I fiddled with the speed sensor connector on that side and it's mostly gone away. To be clear, I did not find any sign of a broken wire which we saw on earlier Impalas that was causing the ABS to kick in.
The traction control firing sound has a much higher repetition rate than the ABS, and sounds a lot like grinding. Also, I could feel it in the steering - when the brake was being applied, it would pull the wheel a tiny bit.
Also, when mine would engage, I don't recall ever seeing anything light up on the instrument cluster.
If you are actually hearing the rears rather than the fronts, it's obviously not due to steering. Not sure what could be affecting the rears.
But on mine, when I jacked it up and removed the wheel, I could see, when the steering wheel was turned, the wire going to the speed sensor on the right front was getting flexed a tiny bit, and I inferred that flexing was affecting the signal thru the connector at the rotor. My take was that the contacts in the connector weren't making a good connection and that was causing extra pulses to be seen by the computer which would interpret them as wheel spin which in turn caused it to apply the brake.
I extracted and inserted the connector several times to clean the contacts, and then made sure it was fully seated when I plugged it back in. The problem went away. I think I recently heard it fire for the first time in months, but it happened so fast, I'm not sure what it was. My son's been driving it since, and he hasn't made any comments about it.
Anyway, if you're hearing a grinding noise occasionally, it may not be rotor rust