Aliens would be the greatest discovery in man's history. There would simply be no way to suppress it. If they can indeed be captured on cameras and sensors, the sheer number of cameras and sensors in the world for the last few decades would guarantee several high definition videos and measurements with no other rational explanation. The greatest minds in science would be clambering over each other to study them.
Sagan also goes to great lengths to explain that there would be very little chance of aliens appearing or behaving the way they purportedly do. For example, they would certainly not fly almost exclusively over North America. The aliens are also not likely to change with the times, as is suggested by the fact that UFO conspiracy theories often follow patterns that change every few years or so. Alien sightings used to be green men in flying saucers, then they left crop circles, now they are little pill-shaped objects.
In short, we lack any evidence at all, many who claim to see them later recant or admit to being charlatans, and the descriptions of aliens are rather unimaginative when compared to the actual possibilities of how alien life may look or behave.