It appears thiefs need only a little water and a cotton cloth to wipe your signature clean off your credit or debit card.
Just moisten your card and let it sit that way for a couple hours. Then, wipe clean. And viola! No more pesky signature to get in the way of identity theft.
How did I discover this? Well, my cell phone took a dive into the Stanislaus River at about midnight on Labor Day and I dove in after it (without removing my wallet, mind you). The next day, I decided to dry off my cards with my shirt as I discovered they were still damp while buying iced tea. And now, to go along with my ruined phone, I have a T-shirt with a beautiful blue streak on the stomach.
So now you know how a thief can easily wipe off your signature. And not to jump into bodies of water with your wallet. Unless you want a stained shirt.