It is very unlikely that a small exposure of rebar like that would cause staining anywhere other than within several inches. Or to put that another way, that kind of rebar problem does cause local staining, like the stains seen in the picture, but it wouldn't cause staining outside the region we can see in the picture.
Copper can come from heaters, other copper plumbing (rare), ionizer or "mineral" systems, and some kinds of algaecide.