5. Nothing
Although it is incredibly doubtful there's a very small chance that when she drove off at the end of episode 4 Carol was bowing out of The Walking Dead for good. Perhaps the shows creators had no choice but to write Melissa McBride out of the show for a period of time but did not want to kill her off in case she returns at a later date. In reality though Carol Peletier has become to important a character in the show to just vanish in such an underwhelming fashion. There is no way any one involved would allow something like that to happen to a character who has meant so much to the 'group' as well as the fans for the last 4 seasons. There has to be more to it than that.