Congrat's on the 2007 RR SC, they are a terrific vehicle!
I agree with "azjulian", also brake pad dust can coat the slide pins and even contact area between pad and caliper piston, causing a squeal, flushing the pad and caliper area with water and a brush to try remove excessive brake dust can sometimes work, failing that, taking the pads out of the caliper and washing the caliper piston area with a stiff brush (not damaging the piston boot), cleaning pad slides/pins with a wire brush/emery tape and replacing the pads can work also. Low speed squeal seems to happen with european vehicles due to the excessive disc pad "dust" from my experience.