Experienced cavers never use flares as: 1) There is the potential for flammable/explosives gases such as methane being present. 2) The flares would litter the cave, the spoiling future cave explorations for others.
One of the characters comments upon the fact "someone might have been there (the cave) before". This is obvious well before he says it as there are clearly several signs along the rocks leading to the cave itself and the cave's entrance is covered what appears to be a heavy welded steel crating.
Correction: The cave is known to exist and has been sealed by the Government. The fact that the seal has been broken, as Adrian sees the mesh has been damaged and tells them that the bolt-cutters will not be needed, implies that someone has been INSIDE the cave since it was sealed.
Correction: The cave is known to exist and has been sealed by the Government. The fact that the seal has been broken, as Adrian sees the mesh has been damaged and tells them that the bolt-cutters will not be needed, implies that someone has been INSIDE the cave since it was sealed.
The 3 characters dive through the cave with only their diving equipment and a spare tank. Then, next thing you know he is using night sights. Pretty sure he didn't sneak them under his wet suit with out his friends seeing them and asking why the hell he needed night sights.
It's extremely doubtful that an experienced cave diver would smoke before a dive. Doing so risks breathing difficulties during the dive and potential respiratory infections afterward.