I heard second hand (through the guy who cuts my hair) that some of the screens do pick up tape now. One of his other customers got his wife to watch the screen as he went through the scanner, and she said the white horseshoe shape of his tape was visible on his head. It wasn't screaming for attention, but it was visible. They didn't touch the guys piece or ask him about it though, so it seems they aren't worried about hairpieces and/or they know to be discreet.

I would rather this didn't show up, but I am not too worried provided there are no awkward questions or touching. I guess if you don't want the issue to arise you could use an all glue attachment.