An apparently metallic or luminous object in the sky at any time after the 1940s could possibly be explained away as experimental terrestrial craft but we have reports of such things going back much further in history.

It's funny how today many people call them "flying saucers" due to a misunderstanding of Kenneth Arnold's description of the objects that he claims to have seen in 1947 because more than 800 years ago, someone reported seeing what he described as a "flying earthenware vessel" in the sky over Edo(Tokyo) Japan.

People have been seeing things in the sky for a very long time. We simply don't know all of them are. Anyone who claims to know with absolute certainty what they are is just speculating. Chances are, that person just wants to sell you their book.