I also had two zeroed e-gift cards, which I unfortunately didn't notice until I got to the desk to pay checking out of the Grand Cypress.
Phone service told me they had been used over the summer at two properties I had never been to (Jersey City and Lake Tahoe)
Hyatt was quick to reissue (the next day) but that didn't help me and I unfortunately had to change my plans to cover the stay with the gift cards.
I agree with the upthread comment that this is likely a brute force (or other) hack, since all a person would need is the number.