Well right .. if the kid is hiding it VERY well.. then yes, there can be times noone notices it. Kids are good at hiding things like that..
But usually those types of kids are monitored pretty closely... or they are supposed to be anyways..
And all that kids "team".., medical doctor, family, school teachers, school counselors, and therapists are notified to keep an eye out for ANY marks on the child and report it.
Usually, if its reported, the child is also brought directly to a medical doctor for an exam also.
This has MY experience with these kids..
I think every state is different, maybe?
ETA: I've also had kids that were required to go to the doctors every so often for an "exam"... just in case they were hiding marks on themselves..