When city slabs could be create from low-atmosphere level we could simply try to lay a new slab on existing hidden-one (as stated in the rules) and city was revealed. Now when city creation is allowed only from ground level such operation is not possible.
We get: "You are already inside a city. Please find an empty terrain to build your city." when clicking "City Creation" option.
There definetly should be an option to reveal a hidden cities either in action menu or as an option after clicking "City Creation"