run editor script when just returning from play mode

The way you do this is to include a script that's set to ExecuteInEditMode in your scene. When you enter and exit play mode, the script's OnDestroy will be called in the mode it is exiting, and it's Awake will be called in the mode it is entering.

Then, you simply check Application.isPlaying in both methods, and save a persistent message (say in PlayerPrefs) saying that the thing should be spawned.

I wrote up a little example script, it's been tested and should work:

 [ExecuteInEditMode]
 public class DoStuffWhenExitPlayMode : MonoBehaviour
 {
 
     void Awake()
     {
         if (!Application.isPlaying)
         {
             Debug.Log("Awake in edit mode");
             if (PlayerPrefs.GetInt("should spawn") == 1)
             {
                 PlayerPrefs.SetInt("should spawn", 0);
                 
                 //Do spawning here, this object shows up in the scene.
                 GameObject exampleObject = new GameObject();
             }
         }
     }
 
     void OnDestroy()
     {
         if (Application.isPlaying)
         {
             Debug.Log("Destroyed in play mode");
             PlayerPrefs.SetInt("should spawn", 1);
         }
     }
 }