I've seen some strange things in my time in the night sky. I've also seen strange things in NASA videos that they've released, such as what would have been considered "space debris" suddenly changing direction and speeding off.
Aliens exist...this is a matter if scientific likelihood. However, whether or not they've visited us? People state that it would be impossible, but that's just narrow mindedness on their part. We used to say that breaking the speed of sound was impossible, or that flying was impossible, but those have since been proven wrong so who says there isn't a way to travel faster than the speed of light.
Yes, many human-made things could be mistaken for alien spacecraft. Here in Bournemouth, just a few years ago, people went on thinking they saw some, just for it to be chinese lanterns. However, the footage from NASA of things from orbit and eyewitness accounts by so many people, many of which match eachother's descriptions (before the Internet), it's hard to ignore.