Excel - display message when missing file reference is found

I can't seem to get this right. I have a client with 10000's physical files, listed in Excel, to which I need to apply barcodes.

However, amongst these are approx 2000 files that should already have a barcode attached but I know won't.

So, I need to somehow configure Excel to display a message when one of the 2000 is found so we know to check our other DB. Hope this makes sense.