"Sponsors have raised privacy concerns relating to a number of [the listed] strategies. DOL acknowledges these concerns and generally recommends ... asking the relevant parties (e.g., beneficiaries, other related plans) to forward letters to the missing participants.... DOL notes that not all of the listed strategies will be appropriate or necessary for every plan. Also, DOL states that fiduciaries can consider 'the size of a participant's accrued benefit and account balance as well as the cost of search efforts.' ... [It] seems probable that evidence of [a] written policy will be required in DOL investigations and similar settings."