It was **Docker for Mac**! 

There is no clear name indication that this is a Docker process, so you can't clearly spot it in the Activity Monitor unless you use `View -> All Processes, Hierarchically` or view the process origin with <kbd>⌘</kbd>+<kbd>I</kbd>.

Stopping Docker releases the resources.

[Github issue][1]

  [1]: https://github.com/docker/for-mac/issues/5812