Hmmm .. I wonder if all those SC day passes that various sites give away or you can buy on eBay or the like are ones that have been used before and given back to the user.

Are you sure they aren't scanning it for some barcode or something ... would seriously sound like a open loophole someone could abuse if they wanted to.
NOTE: I've never had a chance to use a day pass as when I've used the SC I've had a card that gets me in (SC Card or Amex)