"[T]he DOL expects that plan administrators will track participants with stale contact information, keep a record of mail that has been returned, and account for checks that are uncashed.... As further suggested by the DOL, when a plan (or just its assets) is acquired in connection with a corporate transaction, the buyer should require the seller to provide records of missing participants and any search efforts that have been taken.... Plan administrators should update any applicable policies and, in some cases, plan documents to include these additional search steps."