And therein lies a major problem - it should be simple for anyone to find the rights owners for any registered work. After all, if we're granting them a government license to control something, then everyone ought to know who has that control.
Chances are, based on experiences with authors here on SOL and elsewhere, if no one responds to such an inquiry, it's likely because the author is indisposed (i.e. no longer able to decide such issues for themselves) or deceased, and the family didn't think their legacy important enough to bother with.
In short, in you put forth the effort to contact the rights holder, and can prove so if questioned, then you're entitled to use the material, provided you state somewhere in the document that you were unable to reach the copyright holder, and are willing to remove it if asked. That's the extent of the legal requirements (providing you don't go over the legal limit, otherwise the courts may decide you decided to take advantage of the situation).
This discussion comes up a LOT when dealing with font, image and other issues, and has stood the test of time. The key is, you can't simply assume you have the legal right, but you've tried to acquire the legal rights to the best of your ability (i.e. you couldn't just 'not find' them, but there was no way for you to contact them.
This typically happens when a bunch of people want to use a publicly accessible font, multiple people have tried contacting the legal owner, on repeated occasions, the font has been publicly available for some time, and no one has ever registered a complaint about its use. Even given all of that, everyone recommends adding the legal disclaimer as well.