In order to pay out the lost/missing participant you must first have written proof that you are unable to locate the participant. You can do this by mailing a notice of benefits to the participant at his last known address via certified mail receipt. The IRS link below provides a sample participant notice which you may use. You should keep a copy of the certified mail receipt and the original deliverable certified mail receipts for your file.
The next step is to use a locator service for all lost/missing participants.
Below is a link to a website of a private company that locates lost participants. Please take a look at their website. This company does not have any minimums, sign up fees, etc. and charges $10 per report. The link is: