Apple Watch 3 causes dots on skin under heart monitor lights

About six months after I bought my wife an Apple Watch she developed two red dots on her wrist directly under the two vertical sensor lights on the back of the watch. She never put skin cream where the watch sat on her wrist as she didn't want to get it dirty. She followed Apple's recommendation and had the band loose enough to fit two fingers between it and her wrist. She cleaned the back regularly with a microfibre cloth. She doesn't have any allergies. She has followed all advice given on this site and to no avail. If she were allergic to the back of the watch then she would have a circle rash the size of the back of the watch. She doesn't, she just has two spots directly under the sensors.

Please do not insult us by providing an answer you have already given as we have explored all of those to no avail.