As mentioned, work-for-hire has to be arranged explicitly and in advance. And while it's very trendy and common right now, thirty to fifty years ago it was not; "parent's wedding photos" were probably not done with any particular concern or contracts about copyright.

If they were done professionally by a studio that specializes in such things, copyright may have been considered. If they were done by "cousin Sarah's brother-in-law Ed, who dabbles in photography"--whose name may not have been written down anywhere--there may be any paperwork at all to track down; there was just a verbal agreement "we provide you all the film you can handle; you take pics; we get pics." The existence of copies may not even have been considered.