I just heard back from the dealer.
Apparently the rear coil springs are shifting when the car makes a turn. So no squeak when it is just an up and down motion driving straight on a bumpy road. You have to be rounding a turn for the springs to shift laterally causing the squeak. It sounds like it may have been associated with the problem explained in:
Subaru TSB 05-72-19: This bulletin addresses isolated concerns of a squeaking / creaking -type sound from the rear suspension. The condition is caused by a burr on the end of the coil spring winding produced with an end gap which was out of specification. This allowed the burr to make contact with the mount. Over time, the burr, which is characteristic result of the manufacturing process, can penetrate the rubber isolator and come into direct contact with the strut mount as shown in the photos below. This contact creates the sound described. Should this occur, both the mount and coil spring must be replaced. NEVER attempt to grind or otherwise alter the burr on the new or original equipment coil spring or repair any contact marks with paint or other coatings. Replace the affected parts with new ones.
They are ordering new ones to be installed next week.