If you play a game with fog of war, and have already discovered some additinal parts of the map, such as new piles of gold or hills blocking your way then these remain shown even when all your units leave this area - the game remembers they are there and continues to display them since they are in the uncovered area of the map.
Now when you save such a game and restore it, this information is gone. You can still see the piles of gold and hills your units are close to, but the other areas you have previously uncovered are again unknown to you. Additional piles of gold and hills which are out of sight of any of your units are no longer visible on the map. To make them visible, you need to send your troops there once again.
For an example, have a look at
this image which shows a reloaded
4hills8kings map (the default eight player map with a circle in each of the corners of the map, each of which contain two bases). The location you are looking at would normally contain a line of hills, forming the top left of the circles. You would also see way more wood next to it (no trees have been cut there, yet). Compare to
this screen where the camera is pointing at the same area but both hills and trees are visible (this is before saving and restoring a game).
