Continuity mistake: In the scene where Lex is running in the abandoned whaler camp, the "shield" she is holding keeps changing hand between shots. (01:18:05)

Alien Vs. Predator (2004)
Plot summary
Directed by: Paul W.S. Anderson
Starring: Lance Henriksen, Ewen Bremner, Sanaa Lathan, Colin Salmon, Raoul Bova
A crew of select specialists have been selected to investigate the discovery of a temple underneath an abandoned whaling station. They venture into the temple only to discover after finding alien eggs and advanced weaponry that this temple was used to worship aliens known as predators, who used this temple for hunting different aliens (known as 'aliens'). The crew take the sacred weapons the predators use for the hunt, causing the predators to pursue them. The crew is under the impression that the Predators wish to kill them, though after many members have been killed by aliens and predators they find that the predators required the weapons to survive the hunt and as such were forced to kill to reclaim them.
Alexa 'Lex' Woods: Where exactly on the ice is this?
Charles Bishop Weyland: It's not on the ice, it's 2000 feet under it.
Question: Can anyone tell me why the Predators didn't scan the 'token' human survivor or the last dead predator to see if either were infected with an Alien? Especially when the Predator was not wearing his face mask which would serve as protection against this infection.
Chosen answer: Given the speed with which the chestbursters emerged from the other humans, it seems likely that these have been engineered by the Predators to emerge as quickly as possible. As such, they probably assumed in Lex's case that, were she impregnated, it would have emerged before that point. As for their fallen comrade, it may well have simply not occurred to them that one of their number would be foolish enough to get himself impregnated. Even proud alien hunters can make mistakes.
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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.