It is always global. add_filter() and add_action() are just wrapper for the global variable $wp_filter. So it doesn’t matter where the function is called.
The same is true for apply_filters(), apply_filters_ref_array(), do_action_ref_array() and do_action(): they work in the global namespace, that’s the reason why the callback handlers (functions or methods) have to be public. A protected class method cannot be called from the global scope.