Go to the scene inside the hole where Liz wakes up and switches on her torch. She shines it at Frankie and Geoff who are asleep, and Mike, who is awake. Liz asks Mike if she can talk to him and he says, "Shall I come over there?" As he is walking over to Liz's bed, you can hear someone coughing. It is very clear, as no-one is talking at the time and the scene is silent apart from the cough. You can tell it isn't Mike because the camera is on him at the time and it isn't Liz because it's coming from further away. It can't be Frankie or Geoff as they are both definitely asleep. This is proved when Mike switches the lights on and you see Geoff and Frankie wake up. It doesn't sound like a cough you might do in your sleep, and even if it was, something that pointless wouldn't be put in to the script, and I think the actors would be able to resist coughing just for a couple of minutes. You can just tell by the way it sounds that it's one of the camera-men or any of the other non-actors.