I don't believe in UFOs
reaching earth (no offense, Martian
), but I do believe in alien life, and perhaps alien life that predates our evolution from bi-pedal primates to what we are today, and thereby, it's feasible they have reached a higher level of intelligence, reason, and technology.
If they were able to reach us, we would know for certain of their existence and their presence wouldn't be such a secret. Roswell wouldn't be a one-off. The pyramids a one-off, Nazca Lines, etc.
Stephen Hawking postulates that one of mankinds biggest threats is that of alien life making it here and exterminating us. Surely, if aliens have the ability to reach us, from such far distances (outside our solar system/Milky Way/galaxy), they would have incredible technology and able to do with us as they pleased.
Now, in the literal sense, yes there are UFOs. Stealth Fighters/Bombers at one point were UFOs. As were B-52s before and during WWll.
Lots of top-secret aerospace technology is not revealed to the public, but must be tested in our skies.