Although it is extremely arrogant to believe that, given the entire universe, only earth is able to sustain life, it is not likely that aliens have visited. With the amount of technology and effort required for inter stellar space travel, there would likely be a need for alien civilizations to visit. If a civilization was to make the effort to visit earth, it would likely want to make its presence known rather than do it in secret.
Alien sightings are fiction and make good material for movie plots. Aliens are not real. There has been no scientific proof of aliens. If aliens existed there would be scientific proof. Aliens are good for entertainment and lively imaginations. They might help stir up some people who are bored, but they are purely fiction.