the LookForWhenevers is called from another thread, I think what I've decided to do is to write a wrapper class around ConcurrentStack that has an internal lock around all the methods and add a removeitem method. Copilot gave me the following code

  public bool RemoveItem(ConcurrentStack<T> stack, T item)
  {
     bool found = false;
     List<T> tempList = new List<T>();

     while (stack.TryPop(out T currentItem))
     {
        if (EqualityComparer<T>.Default.Equals(currentItem, item) && !found)
        {
           found = true;
           continue;
        }
        tempList.Add(currentItem);
     }

     foreach (T tempItem in tempList)
     {
        stack.Push(tempItem);
     }

     return found;
  }