I don't think he meant FRS
I suspect he meant that his eyes gave him away by moving around the table to observe the cards. That is one tell that pit bosses look for, but it doesn't mean much by itself, since a lot of players watch the hands of other players. I'd never be able to do the sunglasses bit myself. I have a hard enough time seeing the cards in the clear, and sunglasses would only make it worse. :-)
I do agree with your comments about giving the pit notice that a card counter is coming downtown at a specific date and will be wearing sunglasses.