Example.cs

using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
internal static class Example
{
    private static readonly Color COLOR = new Color32( 40, 40, 40, 255 );

    static Example()
    {
        EditorApplication.hierarchyWindowItemOnGUI += OnGUI;
    }

    private static void OnGUI( int instanceID, Rect rect )
    {
        var xMax = rect.xMax;

        rect.x      =  32;
        rect.xMax   =  xMax + 16;
        rect.y      += 15;
        rect.height =  1;

        EditorGUI.DrawRect( rect, COLOR );
    }
}