Trenches

[PoorOldSpike]

Clear summer day, a line of infantry approaches from the right. They can't see any trenches yet..
This series of test pics show how trenches are always spotted at around 200m (closer in dull weather or night) even if they're in terrain so long as there's LOS to them.
(Incidentally the grid is 20-metre squares)
Once spotted, an attacker can rain artillery or tank area-fire on them to try to flush out the occupant even though he can't see if anybody's even in there. (In fact leaving a few trenches empty is a common CM tactic to make an attacker waste time and ammo shooting at them or avoiding them)
Also note the excellent cover trenches give to the occupant, the enemy can't see him until he closes to inside about 20 metres.