Yeah but if the frame is actually cracked it will most likely show up at the surface. since the surface will have to flex slightly more than the more center of the tube, since its in the periphery, bending/flexing is done more at the periphery no matter what tube shape. so its more likely its cracked at the surface and not in the "center" than the other way around. by a factor of like 3,25 million or so.
Only time i have seen stuff cracked under the surface not really showing surface damage is when there was "blunt force truma" and then sometimes the paint is so flexible so it follows the cracked carbon (not very likely though) without indicating any damage. but if you simply squeeze this part it will definitely show up as delamiated. since its soft and weak there. and there will always be some sign of it. otherwise the damage wouldn't have happened in the first place. you just have to be looking carefully. I dont usually work with carbon but i do look for cracks and similar all day long at my job.
I understand they shops want their backs free. but what about the customer??
if you were to send in a scratched frame to the lbs and they send it away. its like 50% chance you wont ever get it back and you will never know why. And i kinda doubt the manufacturers local offices are set up for any kind of hitech non destructive testing. I'm guessing they are just as good as the lbs for determining this.
I however am very proficient in destructive and non destructive testing of anything.
I have have been trained/educated in a fighter plane factory by some of the best carbon guys on this planet, from the lab, so i feel 100% confident inspecting stuff myself. Not all people may be like that though.