Continuity mistake: Towards the beginning, in the scene where they are first going down the tunnel, and the one guy slides out of control. When the woman uses her pick axe to stop him, she slams it into the tunnel before he passes, and his jacket catches on the ax. In the next shot, the axe has struck the jacket in the middle, the jacket is no longer "hooked" on the ax.
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.