C# Trouble testing whether a list of game objects contains a specific game object

Hello. First time help seeker, and I greatly appreciate any help you guys can give.

I want to build a list of game objects based on a certain criteria. To keep this example simple, let's say that this list will be game objects that are within sight of the player at a specific frame X. Then I want to later test a selected game object to see if it was spotted by the player during frame X.

Here's a simplified version of the code:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class PlayerControls : MonoBehaviour {
 
      public List <GameObject> spotted = new List<GameObject>();
      public GameObject enemyObject; // starts as null, used to hold a specific
                                     // object for testing, then set to null again
 
      void ScanArea()
      {
           RaycastHit hit;
 
           if (Physics.Linecast(player.position, target1.position, out hit))
           {                        
                if (currentHit.collider.gameObject.tag == "Enemy")
                {
                     spotted.Add(currentHit.collider.gameObject);
                }
           }
           // multiple targets will be linecasted in this way
      }
 
      void TestGameObject()
      {
           enemyObject = // some object that is going to be tested
 
           foreach (GameObject obj in spotted)
           {
                if (enemyObject == obj)
                {
                     DoSomething();
                }
           }
      }
 }
 

What is happening is that the test works when I test the object that happened to be the first object added to the list "spotted" but if I test any of the other objects that I know are in that list, they do not work.

I have used a workaround where I alter the color of spotted objects then test for color when I do my test, but that seems unnecessary and unideal. I'm sure I'm missing something obvious but I can't seem to figure out what :X

EDIT: looks like this is the error that might be causing me troubles: "InvalidOperationException: Collection was modified; enumeration operation may not execute." I'm not really sure why I'm getting this error though.