Xavier realized that Sierra was hard enough to watch over, even without the added danger of the serial killer’s threat hanging over her head. He definitely had his work cut out for him; especially how independent she is. The only true way of knowing, exactly, what the killer was up to was to get all the information. Once he had a nice chat with Agent Miller, he finally understood just what he was up against. He also knew that to keep Sierra safe he needed to help the FBI apprehend this psycho any way he could. He wasn’t ready to leave Sierra alone but what he was about to do next he didn’t want her to see him…yet. He waited in his hiding spot a little while longer just until he was sure he could leave her.
The sudden darkness of her apartment let him know that it was safe to leave.
