There's lots of missing people that haven't been found. Of course, there's going to be a certain percentage of people that get abducted and killed by space aliens. Its not only entirely possible, but just like there's a probability that you'll die in a car accident there's a probability for getting abducted and killed. But aliens don't have to abduct you to kill you. They could just be driving around and accidentally hit you with whatever they are driving and speed off and disappear around the corner. Or they could determine you know too much, and then kill you in a number of ways.