I think it’s quite possible that certain Secret Service policies and procedures have been classified as Secret or above, but I don’t know whether that actually is the case or not. I doubt anything as generic as “Take the President to the bunker when there’s trouble” is anything that would be classified.
ETA: And if a Secret Service agent did disclose actual classified information, whether it amounted to a crime seems to depend upon the nature of the information. The criminal laws are targeted specifically at defense and foreign intelligence info, not the President’s lunch order.