update. took it to sentry west in shrewsbury, ma and I'm glad I did once again. (sorry this update is a little scatter brain thought process)
The rotor for the most part was really smooth. Which confused me because the portion i checked had a groove. After they showed me the brake pad and it had failed. It actually cracked in the center and then was dragging on the rotor in the center. They told me it could be due to excessive heat possibly and the pad cracked under the heat and suggested things like rotor going to fast otherwise excessive speed or something may got caught there and heated up the pad. I found it a little hard to believe as it was a rear pad and which has a small percentage of the braking force and only one of the rear pads were affected. For all we know the pad installed incorrectly and it was dragging which caused a thermal crack. Or something stuck and then it cracked. Nonetheless the pad clearly had failed.
For the good part. They said normally the pad and such are only covered under 12,000 mile warranty as its a wear item; I'm well past but they said they would fix it free of charge and they did no cost to me. Part of me thinks they were a little worried I'd complain that they didn't catch it the other three times my car was in this month or that it was installed wrong. Whatever the reason the pad definitely failed, installed wrong? Manf defect? Foreign object? Mech failure? who knows.... But Sentry did me right and I'm very pleased with them once again.