Cast iron moisture on inside top
This happened to my Cast Iron when I left it in the oven. If it has the slightest hint of moisture, it will rust, even if it is seasoned. You don't ever want a cast iron or Wok to be left sitting around with moisture on it.

Scour your cast iron with a bristle-brush to remove the rust. Use hot water. Wipe it dry with paper towels. Keep washing until the paper towels aren't showing any brown-orange from the rust.

The next part is crucial!

Take a dry paper towel. Put a small amount of oil in the pan. Coat the entire surface of the cast iron with oil. This layer of oil will prevent any further rust from forming