Continuity mistake: When Lex comes climbing over the edge she rams one of her icepicks in the ground. Two shots later she has suddenly rammed her other icepick in the ground as well. We would have noticed if she did so. (00:04:25)
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.