تعریف و معنای "bubble under the radar"

What is the origin of the idiom "bubble under the radar" and when to use it?

The idiom "bubble under the radar" is thought to have originated in the military, where radar is used to detect objects that are flying or moving. If an object is flying below the radar, it will not be detected by the radar system and will therefore be able to go undetected. In the same way, something that is "bubbling under the radar" is happening or developing but is not being noticed or acknowledged by others. It is used to describe a situation where something or someone is about to achieve success or popularity but has not quite reached that level.