There must be some sort of provision for dealing with this because we found a KTTW card that a passenger lost several weeks before on CC. I've heard of other folks losing theirs as well.
I almost forgot, but on out last cruise, on the last nite, in a rush to get the bags in the hall before the bell tolls... my DW packed her KTTW card in the suitcase accidentially and the suitcase was gone before she realized it. Customer service replaced on the spot that evening, but she really didn't need it again.