Sensor on the down pipe or the catback?
Sensor on the downpipe could be for a aftermarket wide band o2 sensor, not sure how familiar with these cars you are but stock the rear o2 sensor would have been in a bung behind the CC in the cat back.
If you havn't had any check engine codes pop up for the rear o2 then it's probably been tuned out already and in that case seeing you don't have e-testing don't worry about the CC as it will not affect how the car runs and improve flow.