Leon flew for one day and one night without resting and ignoring all the monsters he saw along the way. He was finally back in New York, but his head was in Las Vegas. The demons probably were already attacking, and his little sister was fighting them...
"Focus, focus... one problem at a time."
While Leon was away, it didn't look like things changed all that much in New York. Several teams were patrolling the borders, and the survivors of the city were using new floodlights to illuminate the skies. At that point in time, it was safe to conclude that Willia, Sanchez, and Alisson decided to stay in the main building.
"This will end tonight... five days have passed since they discovered my goals, I can't give them any more time to do as they please or plot anything."