The potential risk with FAA records is advancing information technology. While they may not have caught now, if the record stays out there it's possible some future linking/consolidation of databases could flag it to their attention. This is a somewhat remote threat, but it has happened recently with FAA aeromedical...they setup a system to match pilot medical records with federal disability claims. Many of those collecting disability had not reported their disability problem to the FAA. Some of them went to jail.
Definitely get it expunged. If you are going to lose sleep, contact an aviation lawyer and ask about reporting this to the FAA. He will know the risks and the process. Other people have reported things after the fact like this, and my understanding is that the FAA generally accepts it in good faith without penalty if it's an honest mistake. But ask a lawyer first.
The good news is that the criminal aspect of falsifying an FAA form probably requires intent, which they will have trouble proving if you self-report the problem.