Hello,
Not sure if this is a bug, or just a curiosity related to team games..
During FFA games, the status of cities in the fog of war (whether razed or re-built) in unknown until you reveal the area from the fog of war. So if an opponent had re-built a city that had been previously razed, you would be ignorant of the change until a new sighting was made into the area.
In a team game, we have seen the status of cities in the fog of war changed from razed to re-built (appears as a Neutral city in the fog), at least this has been the case in current game 80506. This creates a situation that you have knowledge of enemy activity in an area without ever revealing the fog (at least in terms of city status).
-Brutus