Where the duties of any state agency for which an appropriation is available make necessary the use of moneys for purposes of a confidential nature, such state agency may, without at the time furnishing vouchers and itemized statements, draw from such appropriation an advance for such confidential purposes. Except for withdrawals by the Attorney General as provided in Section 12572 of this code, the total amount allowed any state agency for such confidential purposes shall not exceed two thousand dollars ($2,000) in any one fiscal year. Claim for such an advance is exempt from Section 925.6 of this code, but shall be accompanied by a certificate of the purpose and necessity for secrecy. The state agency shall remain fully accountable for any amount thus advanced, and when the reason for the secrecy no longer exists, or, in any event, within one year after such amount is withdrawn, must account therefor to the Controller who shall require vouchers in substantiation of all disbursements and make such audit as may be necessary.