All of the various ways the agencies have proposed procedures or made suggestions about finding a missing participant, no one explicitly tells us what to do when, for a plan that is not terminating, we try everything in good faith and cannot locate the missing participant - with some exceptions that are not considered acceptable by all of the agencies.
The IRS says we can forfeit the benefits and pay it if the participant is found. The DOL doesn't like this.
The IRS at least acknowledges in the 5500 instructions to the compliance question about benefit due but not paid that some participants will not be found by saying: "Note: In the absence of other guidance, filers do not need to report on this line unpaid required minimum distribution (RMD) amounts for participants who have retired or separated from service, or their beneficiaries, who cannot be located after reasonable efforts or where the plan is in the process of engaging in such reasonable efforts at the end of the plan year reporting period."
There does seem to be some grudging, temporary acceptance by the DOL of the notion to write the check and let it escheat to the state. The IRS doesn't seem to mind as long as they get default withholding.
The IRS and DOL want us to report to them people due benefits, but neither agency wants to let a plan cease to have the responsibility to find a missing participant (unless the plan can pass off that responsibility to a business that will take the participant's account where it slowly disappears into the abyss of administrative fees).
The PBGC is willing to help, but not for DC plans unless the plan is terminated.
Hopefully, sometime, we will have clarity across all agencies about what a plan can do with benefits due to participants who remain missing despite everyone's best efforts to locate them. This has about the same chance as there being universal peace, love and happiness.