Hard to say without photos to peek at...

Do the marks fade away by a later point in the day?
Are you feeding live or frozen?

My guesses if there's no real way to injure themself is that live food is doing it, or it's just the little skin indents and creases they sometimes get from being balled up in a hide, but because it's white belly scales the color is noticable.

The only other thing I could think of, is I've seen "red scratches" on a snake that was self harming. He would panic and bite at himself. In that case, when the snake stressed from handling and even a bedding change sometimes, it would bite at itself.