I've noticed these "suspicious" blemishes on Ruby's plastron, but I wast sure what they were. The spots really are quite small, and they haven't gotten any bigger. Plus, I don't think her enclosure is humid enough for shell rot.
Good old athletes foot cream.
It will do NO harm.
It doesn't look like much to worry about, but if it is fungal, it will grow larger.
BTW. The type you get at the dollar store has the same active ingredient as the more expensive drug store brands.
I keep dollar store athletes foot cream and dollar store antibiotic ointment in my tortoise "doctor" kit. Just in case.