Factual error: When they're first entering the whaling camp, a penguin startles Miller. That penguin was a black-footed penguin, which is a bird native to much warmer climates. If that little guy had actually been in Antarctica, he would have died within minutes. (00:20:40)
Factual error: The expedition leader claims that Bouvet Island is not under any national control, because of the Antarctic Treaty. However, because of its remoteness from the continent of Antarctica, Bouvet Island is actually not included in this treaty and Norway has full ownership of this island. (00:12:35)
Factual error: When arriving in Antarctica, there is dialogue about the full moon (la luna di cacciatore, etc...) There is no full moon on 10/10/2004 in Antarctica. But it does look nice in the movie... (00:12:15)
Factual error: The Desert Eagle that Miller & Lex use to shoot Aliens can hold a maximum of 9 rounds in .357 Magnum. Yet they fire quite a large number of shots from the same gun without reloading. High-capacity magazines are unavailable for the Desert Eagle as the magnum rounds are just too big. (01:02:45 - 01:14:00)
Factual error: The members of the expedition are wearing their Snow Goggles on their hats with the lenses to the front. However, anyone who has ever worked in a cold weather environment (especially the guide and her "7 seasons on the ice") would know that the goggles should be worn backwards if they're not over your eyes. This keeps them from getting moisture on them.
Factual error: The movie takes place in the first half of October which is the beginning of the "Antarctic summer" not that far away from the Antarctic circle. That means there should be a lot of sunlight and sun should only set for couple of hours. However all the exterior scenes in Antarctica are shot in darkness.
Factual error: When the predator starts to kill Weyland, Weyland uses his oxygen tank as a makeshift flame thrower. Contrary to popular belief, oxygen does NOT burn. It only allows other things to burn. The three things needed for a fire are oxygen (which he had), fuel (which he did NOT have) and heat (from the flare he lit). (00:59:25)
Factual error: Lex reaches the top of Lho La Ice Fall (Nepal) to find Max there standing in front of an Eurocopter AS350 helicopter. The problem is that this helicopter has a service ceiling of about 15,000 ft., and Lho La Ice Fall is at almost 20,000 ft. This is far beyond the capabilities of the helicopter's performance. (00:03:18)
Answer: When they emerged from the underground pyramid unscathed, the Predators assumed they were not infected. Any Predator, young or old, would have been taught to self destruct rather than be caught or killed. As seen in the previous movies. Lex was given the mark of a warrior, they assumed she was taught their code of honor.