Unity CommentComponent

CommentComponent.cs

using System.Collections;
using UnityEngine;

/// <summary>
/// Simple component that allows users to write comments to place on MonoBehaviours. Mainly for testmaps.
/// </summary>
public class CommentComponent : MonoBehaviour
{
    [TextArea(3, 10)]
    [SerializeField]
    [Tooltip("Enter comment text here")]
    private string text;

    /// <summary>
    /// Gets or sets the body of the comment, mainly used for display of information on test maps.
    /// </summary>
    /// <value>The body.</value>
    public string Text
    {
        get => this.text;
        set => this.text = value;
    }
}

CommentHierarchyIcon.cs

using UnityEditor;
using UnityEngine;
using RedBlueGames.Tools;

/// <summary>
/// Draws a Comment Icon on GameObjects in the Hierarchy that contain the Comment component.
/// </summary>
[InitializeOnLoad]
public class CommentHierarchyIcon
{
    private static readonly Texture2D Icon;
    private static readonly Texture2D ParentIcon;

    static CommentHierarchyIcon()
    {
        Icon = AssetDatabase.LoadAssetAtPath("Assets/Gizmos/CommentIcon.png", typeof(Texture2D)) as Texture2D;
        ParentIcon = AssetDatabase.LoadAssetAtPath("Assets/Gizmos/CommentIcon_Parent.png", typeof(Texture2D)) as Texture2D;

        if (Icon == null)
        {
            return;
        } 

        EditorApplication.hierarchyWindowItemOnGUI += DrawIconOnWindowItem;
    }

    private static void DrawIconOnWindowItem(int instanceID, Rect rect)
    {
        if (Icon == null)
        {
            return;
        }

        GameObject gameObject = EditorUtility.InstanceIDToObject(instanceID) as GameObject;

        if (gameObject == null)
        {
            return;
        }

        var comments = gameObject.GetComponentsInChildrenPooled<CommentComponent>();
        foreach (var comment in comments)
        {
            if (!string.IsNullOrEmpty(comment.Text))
            {
                bool commentIsOnThisObject = comment.gameObject.Equals(gameObject);
                if (commentIsOnThisObject)
                {
                    DrawIcon(Icon, rect, comment.Text);
                    break;
                }
                else
                {
                    if (ParentIcon != null)
                    {
                        // Could eventually use HierarchyProperty (and Find / IsExpanded) to only show this icon when collapsed)
                        DrawIcon(ParentIcon, rect, string.Empty);
                        break;
                    }
                }
            }
        }
        comments.Free();
    }

    private static void DrawIcon(Texture2D icon, Rect rect, string tooltip)
    {
        float iconWidth = 15;
        EditorGUIUtility.SetIconSize(new Vector2(iconWidth, iconWidth));
        var padding = new Vector2(20, 0);
        var iconDrawRect = new Rect(rect.xMax - (iconWidth + padding.x), rect.yMin, rect.width, rect.height);
        var iconGUIContent = new GUIContent(icon, tooltip);
        EditorGUI.LabelField(iconDrawRect, iconGUIContent);
        EditorGUIUtility.SetIconSize(Vector2.zero);
    }
}

Note this has a few utility dependencies that are not imported.