It is not uncommon for employees to leave a company without providing a forwarding address or any contact information, but as a Fiduciary Administrator, it is still your responsibility to use all means at your disposal to locate these individuals.
Since the IRS and SSA have discontinued these services, the Department of Labor (DOL) has provided some steps that may be taken to assist in locating these individuals for any final wages, retirement wages or other payments due them.
It is recommended to start simple and with as little cost as necessary. Begin with mailing a notice to the old contact information of the employee. Other options are to check related plans, such as Health plans or employer records. You can also try contacting the plan beneficiary. If your search returns no results, your last effort should be use the Internet to scour search engines, public record databases, real estate taxes, obituaries and social media.
If you do find the employee and costs were incurred in the search process, you can charge the individual a fee to help recoup those additional charges. Make sure you are consistent with the terms of the plan when requiring those fees to avoid questioning down the line.