Update.. I took the car back to the garage where it was certified and I inspected each of the rotors. All pads were replaced with aftermarket parts. The rear rotors are new and show no sign of wear. The front rotors have slight ridges on the inner and outer edges, but pass the safety. Next, I took the car to the local Volvo dealer and was told the anti-vibration shims are missing on the rear calipers. Apparently the back plates of the aftermarket pads are too thick to accomodate the stock shims. The dealer said they've seen this before and it isn't a saftey issue. I can get rid of the squeek by installing stock Volvo pads and shims on the back. However, the squeek seems to have gone away by itself, so I left the aftermarket pads on there without shims. At least if the squeek comes back I'll know how to fix it.