Fog

This has probably been brought up but i'll say it just in case. When playing some games i have noticed that some of my ships 'disappear' but the screen still moves to its location only there are patches of fog (or shroud or whatever) that cover some areas up.
If i try to send a ship into that area the ship just disappears in the fog. When i then try to move the ship out again it appears when it comes out of the patch.
The only way of removing this is by saving, exiting and loading it again.

Is this a bug or what?