Rigel was left behind because Aru and Uruka were having a private conversation about what Aru had said earlier. He read more about Lysra and Almari from the remaining documents and archives that were scattered on the ground. Unfortunately, he didn't get much information about those 2 names because the documents and archives were barely readable.
While he was checking every wall and shelf to find a secret room, he saw Aru running out of the room. He watched Aru leave and use the teleporter, and then he saw Uruka scratch his head in frustration. He could see that the conversation didn't go well, and nobody was winning in the argument those 2 were having.
"That child. I can't make her listen at all..." Uruka sighed as he walked toward Rigel. "I don't know what you told her or what motivated her to do this, but she's insisted on leaving the village," he explained as he crossed his arms, thinking of a way to convince Aru to reconsider her decision.