@david2b this is completely normal and nothing to worry about whatsoever. What you're seeing is the ultrasonic fingerprint sensor that is right beneath the display. It's normal to be able to see it slightly under the right lighting with the display off.
Is it possible that you have a factory installed screen protector on the phone?
Galaxy S21 Ultra 512g(VZW), Galaxy Book (original, VZW), Tab S4, Galaxy Watch3 (VZW)