During the beginning of the movie, Paul ends up disappearing into the darkness like most everyone else. However, Luke ends up finding him, pretty beat up, in a bus stop lit by solar powered lights. Why did Paul reappear?
According to Wikipedia:
Paul reveals that he had been taken by the darkness, but the headlight that he had on had activated and he had found himself in the street three days later.
and IMDB gives a little more detail:
Paul (John Leguizamo) says that his light went off, and that he was hit by somebody from the darkness. He was taken somewhere else when suddenly his light went on again, so that he doesn't even know how he ended up at that bus shelter.