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.
Continuity mistake: Lex gets the Predators glowing green blood splashed on her clothes when the alien kills it. Next shot her clothes are clean.
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.