When a plane's transponder shuts off?

As I watched CNN talking about what happens when a transponder is shut off on a passenger or other plane it made me realize that this actually gives more credence to the existence of UFOS.

If the only way to be sure what you are looking at on a radar is to have a transponder which reads out what plane this is, what altitude it is flying at and whether it is being hijacked or not (just in code on the radar) when this is shut off all you have is a dot on the screen. You don't know if this dot is a plane. You don't know if this dot is a ship. You don't know if it is the flying dutchman on the ocean, you don't know if it is a UFO etc.

So, if all you have is a dot on a radar screen you have no idea what is it other than if it is moving. At speeds above about 40 knots max you know it has to be in the air. But, you still don't know if it is an aircraft or a missreading of the radar because of a glitch.

So, without an operational transponder aboard and turned on anything in the air people don't know what it is unless someone  radios from it to someone who recognizes the plane as a designation or sees it from the ground.

One expert was asked if this ship might not have been trying to avoid detection and if it could have accidentally avoided detection. The expert's answer was "Yes".