Example.cs

using Kogane;
using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
internal static class Example
{
    private const float  SIZE = 20;
    private const string NAME = "d_console.warnicon";

    private static GUIContent m_icon;

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

    private static void OnGUI( int instanceID, Rect rect )
    {
        var gameObject = EditorUtility.InstanceIDToObject( instanceID ) as GameObject;

        if ( gameObject == null ) return;

        var hasMissingReference = MissingReferenceUtils.HasMissingReference( gameObject );

        if ( !hasMissingReference ) return;

        rect.x      += rect.width - 2;
        rect.y      -= 2;
        rect.width  =  SIZE;
        rect.height =  SIZE;

        if ( m_icon == null )
        {
            m_icon = EditorGUIUtility.IconContent( NAME );
        }

        GUI.Label( rect, m_icon );
    }
}