I have had to deal with this several times over the last year. About one year ago, I found my mother's Wedding Portrait taken in 1958. I scanned and restored it as well as I could, and would like to have high quality prints made. Most reputable labs refuse to print copyrighted material. My preference would be to track down the original photographer and have new prints made from the original negatives, but this has not been easy.
More recently, I was given a photocopy of a portrait taken of my grandmother in the 1920's. I have done the best I can to create a usable image, but printing may be an issue. Depending on when the photographer died, this photo may (or may not) be in the Public Domain. Google searches have been fruitless, other than an photograph of his studio taken in the 1910's.
Most likely, I could use either of these photographs with impunity. It is extremely unlikely that the descendants of the photographers in question would recognize either image and take action. But many professional labs will refuse to print them. Unfortunately, in most cases, the law doesn't make provisions for abandoned or orphaned works.