This is probably the biggest downside to larger levels and exploration in general in GM mode. Fog of war is a must-have feature for running exciting, immersive campaigns because without fog of war players can see every encounter before they arrive at it. Right now the only way to work around this is to port players between areas but that is also limiting.
For hiding encounters or monsters ahead of party you can use "Deactivate Encounter" option from Encounter panel -> right click on encounter header in Encounter panel. Or hide them one by one with right click menu on particular npc and choosing "Deactivate" This should solve issue with players sneak picking encounters in front of them
