We typically don't see this if the plan has been terminated for a while. Usually when amounts do appear well after the termination and final reporting of the plan, the amounts are attributable to a settlement of litigation. It would be absurd to resurrect the plan, update for recent legislation, pass around some pennies, make payments, amend the prior final 5500, prepare a few more 5500s for the intervening years and file another final 5500, send SAR to participants, ...
Let's get real and not overthink it. The trustees or plan sponsor should ask the brokerage firm to close the account and write off the amount. If the brokerage firm adamantly refuses, then one of the former service providers likely will be willing to send an invoice to the brokerage firm to close out the account.