Bob Costas hair used to be thick but it thins down as he gets older. So, when his hair regrows overnight rumors of Bob getting a hair transplant started. Although we want to think that he had hairs planted in his head, there are few factors that contradict the idea.
If you focus on the side of his head, the balding pattern is still noticeable and he has the same hairline. His new hair we suspect is a hairpiece and not a result of hair transplant.