Yes, I have seen many UFOs.

Remember that UFO literally means Unidentified Flying Object.

I have often seen birds flying in the sky and been unable to identify what species they were. 

I have often seen satellites moving through the night sky and looking like moving stars. I was unable to identify them precisely because I did not know to what country they belonged, or who launched them.

I saw something really weird once. It was little lights moving in cloudy conditions. I expect they were those Chinese lanterns people like to launch. People, please stop launching those things. They are a fire hazard wherever they land. You wouldn't like one landing on your field / your garden / your roof and setting the place alight. Helium balloons are no better. They are basically litter. Wherever they land, they can be a danger to animals, who might try to eat them. It ought to be illegal to send litter into the sky, just as it is illegal to drop litter on the ground.