Addition to the Limited Visibility section in the Core Rules:

Spotting in Concealment Terrain
 Tanks that move into concealment terrain and stop prior to being spotted by
enemy units have essentially gained HIP status until one of the following
conditions are met. Motion Status negates this.
 The "concealed" tank performs some concealment loss activity per ASL.
 An enemy units approaches the "concealed" units at a certain range as 
detailed below:
  A Very Large target will be spotted at 12 hexes range.
  A Large target will be spotted at 8 hexes range.
  An Average target will be spotted at 4 hexes range.
  A Small target will be spotted at 2 hex range.
  A Very Small target will be spotted at 1 hex range.
 These spotting ranges are reduced by 1 hex if the spotting unit is BU and
the "concealed" unit is in the side or rear covered arc of the spotting unit.
 Once spotted, the previously hidden tank is not considered concealed for
purposes of C6 Case K and need not be fired at using Area Fire.