I can speak from experience here from dealing with elderly parents and in-laws. I've been in your exact situation.
It is possible that the gentleman actually redeemed the CD 15 years ago, but now does not remember that fact and is "looking for his money".
As our loved ones age, they forget about the things they did when they were 100% and no one else (meaning us kids) was looking over their shoulder. Remember someone elderly today, was not so elderly 15 years ago.
Another interesting item I've experienced is finding stock certificates when going through my grandparents papers. We all got very excited (no, they weren't AT&T) and started tracking them down. After weeks of research, we discovered that they were from a company that no longer existed and were more worthless than wallpaper. My grandfather kept them as a reminder to be more prudent in his stock purchases.
Good luck, maybe the gentleman cashed the CD and put the cash under the mattress.
