As other people have said prior, to reveal a city you need to have landed on it, and there will be a "Reveal City" button where the "Build City" button is located at normally. You press that and it will show it on the map.
If this is in regards to the swamp moon you overpaid for, it is uncontrolled. Anyone can put a city on there that can get past the shields, or the hiddens could have been made before the shield city was put up. Or you got scammed by the person that sold it to you. Being there are so many ways cities could be there, I would investigate those before you cry bug.