In the movie - when Watney is recovering after being left behind and drilled by the antenna - he's laying in his bunk moping. Or thinking. Or both. And you can see every couple of seconds the exterior searchlight flashing through the hab windows as it sweeps by. Even when he wasn't the only human on the planet, when his crewmates were there with him ... what was the purpose of the 24/7 searchlight?
No crew was going to go wandering around on the surface during the night (unsafe! NASA wouldn't permit it!) and meanwhile the light constantly flashing in the windows would disturb the sleep of the crew (leads to unfit crew! therefore to unsafe conditions! NASA shouldn't've permitted it!)
(Or am I overthinking this and this is just one of those movie things. For visualization purposes.)