What are these scratches?

Saw my husband in the shower this morning and he had these scratches on his back. Asked him about it, said he didn't know anything about it, maybe he acratched himself. So I asked him to reach back and scratch his back, hard, and he did. Not a mark. He has no explanation, I know it's not from me, only conclusion is it's from someone else. There has been cheating in our past, thought we were past it and in a good place. Anyone else deal with this? What did you do? I'm so upset...

What are these scratches?
What are these scratches?
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