Gu Ding did not detect Yarrow's wound. He was a pharmacologist, not a doctor. Moreover, Yarrow had concealed it very well. At the very least, he had successfully fooled more than forty of his crew. If his crew of more than forty people, who spent every day with him, were unable to tell he was wounded, it was only natural that Gu Ding would not notice it.
Neptune was the one who had detected it.
Neptune has the ability to analyze a person's physical condition based on the magnetic field surrounding the individual as long as they were within Neptune's range. The magnetic field around Yarrow was erratic and his power signatures were not at that of a true Rank 11 Gene Body. In addition to that, Neptune was more than 96% sure that Yarrow had been wounded based on the video of his fight with Kohld Blood, and he had yet to recover.
