The standard reply form OP's dad received denying his appeal/investigation which stated, essentially, that whoever withdrew the funds had proper authority . . . OR the funds "were used to pay an obligation owed by you."

Could this be the result of a sloppy court judgment execution on the account? For example, if Dad got sued by somebody for something (doesn't matter what) and he lost or ignored the lawsuit and had judgment entered against him for $20,000 and hasn't paid, the person who won the lawsuit can execute on bank accounts and drain the funds without permission, or even knowledge of, the account holder.

Is it possible that the money was taken out for something similar to a judgment execution? Back-due taxes and other gov't. liiens, as well as back-due Child Support Payments are also collected this way, which may explain the multiple withdrawals - if each withdrawal represented a separate payment order or penalty. Maybe Dad isn't telling OP the whole story or he doesn't know that has outstanding judgments?