Not sure exactly how this works - since I don't have a criminal mind (haha), but I saw a news segment which warned that some low-lifes will
go into a store;
take some cards; (evidently, the unloaded cards won't trigger any theft alarms)
peel off the stickers;
write the numbers down;
apply a new sticker and
put the card back on the shelf.
They periodically check the status of the card number online and if someone has bought and loaded the it, the low-lifes already have the card number and security number to go online, register the card and use the balance.
Evidently, you can buy little sticky type tape that looks like the original tape.