I had my Sim hang out in the cemetary and, sure enough, every ghost who came out for the night went straight to the street and left in a car. There were times they'd drive before, but it wasn't as constant as it is now. Out of five ghosts, maybe one would leave in a car and the rest would stay at the cemetary. I believe this car thing is caused by Ambitions and/or the accompanying update. I don't like it, personally. I'd rather have them haunting the cemetary as proper ghosts should.