29 C.F.R. § 4050.104 – Diligent search.

(a) Search requirement. The plan administrator of a subpart A plan must, within the time frame described in paragraph (d) of this section, have diligently searched for each distributee of the plan whose location the plan administrator does not know with reasonable certainty upon close-out, using one of the following two methods:
1. (1) For any distributee, regardless of the size of the distributee's benefit, the commercial locator service method described in paragraph (b) of this section; or
2. (2) For a distributee whose normal retirement benefit is not more than $50 per month, the records search method described in paragraph (c) of this section.
(b) Commercial locator service method—(1) In general. Using the commercial locator service method means paying a commercial locator service to search for information to locate a distributee.
(2) Meaning of “commercial locator service.” For purposes of this section, a commercial locator service is a business that holds itself out as a finder of lost persons for compensation using information from a database maintained by a consumer reporting agency (as defined in 15 U.S.C. 1681a(f)).
(c) Records search method—(1) In general. Using the records search method means searching for information to locate a distributee by doing all of the following to the extent reasonably feasible and affordable:
(i) Searching the records of the plan for information to locate the distributee.
(ii) Searching the records of the plan's contributing sponsor that is the most recent employer of the distributee for information to locate the distributee.
(iii) Searching the records of each retirement or welfare plan of the plan's contributing sponsor in which the distributee was a participant for information to locate the distributee.
(iv) Contacting each beneficiary of the distributee identified from the records referred to in paragraphs (c)(1)(i), (ii), and (iii) of this section for information to locate the distributee.
(v) Using an internet search method for which no fee is charged, such as a search engine, a network database, a public record database (such as those for licenses, mortgages, and real estate taxes) or a “social media” website.
(2) Limits on method. For purposes of this section—
1. (i) Searching is not feasible to the extent that, as a practical matter, it is thwarted by legal or practical lack of access to records, and
2. (ii) Searching is not affordable to the extent that the cost of searching (including the value of labor) is more than a reasonable fraction of the benefit of the distributee being searched for. In no event would searching need to be pursued beyond the point where the cost equals the value of the benefit.
3. (d) Time frame. A search for a distributee under this section must have been made within nine months before a filing is made under § 4050.105 identifying the distributee as a missing distributee.