Wow! I don't know about in California, but we don't get wild mus musculus here. We just get deer mice (Peromyscus sp.), which look a great deal like what you're describing, but are not only a difference species, but really only distantly related to domestic mice. If they're second-generation with a wild mouse, you wouldn't get the spotting (s is recessive), but if they were third gen, you definitely could.