I can give you an example of what I've seen, in Rocky Mountain National Park. On more than one occasion, in Moraine Park (the most desired CG), I've seen empty sites for the entire time that I was there. The sign at the entrance said that the CG was full, and there were reservation tags on the sites mentioned. No one showed up, and the reservation dates were exactly the same as mine. Pity, really. The sites went unused and someone, such as yourself, would have benefited.
My suggestion would be to actually call where you want to stay and simply ask them what their policy is for no shows -- how long do they hold them until they become available to first come/first served.
