You might be just seeing the actual finger print sensor within the screen. Since it has to scan through the screen, if it can see you, you should be able to see it in perfect conditions. As long as your glass doesn't have any scratches or visible damage, thats my best guess
Kinda hard to say much more as you've said, pictures of it don't work. Don't really have any other ideas what it could be if not the fingerprint scanner
Don't worry. It's just the S25's fingerprint sensor. This time, the sensor is much closer to the screen panel than in older models—making it more visible. I researched this a few months ago on these forums for someone who had the same question.