Gents:
It depends on the time period.
WW2 - I agree there should be night disruption. Units that leave a road or path can certainly "get lost" and wander around. I believe 30% is an appropriate setting. It penalizes a player who takes to the fields, but there is a 70% chance per hex traveled that units will remain undistributed. So, a gambling player "could" try to maneuver a unit off road for a few hexes in order to be in a more favorable position come day break.
Modern era (MC) - There should be no night disruption. Units are equipped to operate at night and have satellite provided images of the terrain and areas where they are operating.
Regards, Mike / "A good plan violently executed now is better than a perfect plan executed next week." - George S. Patton /