First make sure there is no moisture still wicking from the pad. Couple quick pokes with the urine probe should answer that question. What kind of carpet is it? If it is wool, then you are dealing with a brown out. Regular nylon carpet could be a mixture of things causing it. Orange deodorizer is a tannin and should help with wicking and brown out. You could try misting straight Orange Deod on it and allowing dwell time followed by 101 (or a 101 mixture heavy on Orange Deod with dwell) on the spot and then clean as usual. If it looks clean when you finish, take a fan and dry it while you are there so you can make sure the yellow doesn’t reappear again. Follow with the probe again to check pad moisture after cleaning. If Orange Deod doesn’t work, you could also try coffee remover.