29 CFR 4050.4 - Diligent search.
(a) Search required. A diligent search must be made for each missing participant before information about the missing participant or payment is submitted to the PBGC pursuant to § 4050.6.
(b) Diligence. A search is a diligent search only if the search -
(1) Begins not more than 6 months before notices of intent to terminate are issued and is carried on in such a manner that if the individual is found, distribution to the individual can reasonably be expected to be made on or before the deemed distribution date;
(2) Includes inquiry of any plan beneficiaries (including alternate payees) of the missing participant whose names and addresses are known to the plan administrator; and
(3) Includes use of a commercial locator service to search for the missing participant (without charge to the missing participant or reduction of the missing participant's plan benefit).