"Lie detectors" are so much snake oil. A pathological liar can self-delude into calmness. At best they are a way to coax a confession. Nontheless some people believe in them with a well nigh religious faith.
What is really interesting is that Eunice overheard these chaps in English. Why would these guys plot in English? Arabic would make a lot more sense if they were serious Muslims.
"Why would these guys plot in English?" Well, this has been hashed out on other threads, but I think the most likely explanation is that they do not share another language in common. Also, 2 of them are naturalized citizens. Isn't it just possible that they don't KNOW another language? I heard one of them speak. He doesn't even have an accent.