Our golden used to get something like that on his belly. I forget what the vet called it, but we washed it off 2x a day with warm water and had an ointment to put on it and it cleared up. He always got it at least 3 times during the summer
I'm not sure, but I suppose it could be some kind of fungal infection. It doesn't look bacterial, since it's not raw or weepy. Could they be scabs from a scrape or hot spot that you didn't see? Theo gets marks like that occasionally, and they feel like scabs. I spray hot spot stuff on them, and they go away after a few days.