Small spots around cat's nipple

Hi, I’ve noticed some small brown spots around my cat’s nipple. I’ve checked her other ones and there are two or three tiny specks on another nipple too, but the rest are clear. I took some photos of the spots and went to see my vet, but they weren’t sure what it was and just advised me to keep an eye on the area. My cat, Wicket, is an indoor cat living in a flat; she doesn’t have fleas, she isn't scratching, and she’s been spayed. Does anyone know what this could be? Kind regards.