The Plan Doc should address unknown, unreachable, incompetent situations, usually they are able to forfeit the monies with the understanding that they are refundable to the participant/beneficiary if or when resolved. A corporate resolution documenting the action, and the actions to be taken upon restitution (assumed earnings, etc.) will suffice. Most likely it was an overfunded safe harbor match, mutual fund's fee redemption, class action settlement payment, or trailing dividends from a closed fund that the prior RK didn't track to a current investment.