Spotting the enemy at night is difficult. Not only is it dark, but the shifting shadows make identification of enemy troops uncertain. Warriors on both sides become confused and disoriented and their performance becomes unpredictable, some failing to fire at nearby enemies whilst others continue to blaze away confidently at long range.
Before a unit fires you must check if they can see the enemy. Roll a 2D6 and multiply the result by 3, giving a number between 6 and 36. This is the number of inches the unit can see. If any enemy are visible then they may be fired at as normal. Note you should only roll for each unit once, not for each model or weapon in a unit. Guess range weapons may fire as normal, but the distance they scatter is doubled if the target is beyond visual range.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): According to the rules for the searchlight, if a vehicle uses it at night it "can be fired on by any enemy units in their next turn…". The enemy must have a valid line of sight to the vehicle with the searchlight; they just don't have to check whether they can see it in the dark.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): A vehicle using a searchlight must roll for distance to see how far it can see in the night. It may then illuminate one enemy in that rolled distance.