Not sure when the walk around inspection should be done with airliners in regard to baggage loading. Im betting somebody should have caught it though.
The structural integrity of the "skin" of an airplane is not something that would get more than a casual glance on most preflight checks. My guess is that this was a ding on a rivet seam that looked like nothing more than a scrape to the baggage handler. Unfortunately, or not, baggage handlers are not required to have degrees in aeronautical engineering. Hey, the pilots and all passengers walked away uninjured. It was a successful flight.