Has this practice been disclosed prior to account opening to each client? If not then I would be very hesitant to touch funds in an account without the express consent of the client, unless it's time to escheat the funds to the state.
Also, you should look at your state's escheat laws. Many clearly state that if an account owner has another active account then that is sufficient to consider all accounts held by that individual as active. Your bank may have a higher standard for returning the account to an active state, but you would still not escheat the account if your state has the above language.