Originally Posted by Galant Koh

If a value is returned it can be left blank.

If a value appears more than once it doesn't matter. The idea of this formula is to check for the existence of a physical item in one or more locations where the item could move between locations. Therefore, if an item exists, it will appear on one of the lists. If it does not appear on the list then it means that it no longer exists and I need to be notified.