It was a couple of years ago, it was virtual cards (I think one was a card given as reward by one of those legit review sites - edit: Gartner or one like those - which will reward you for providing reviews, and the other was money I had elsewhere that I converted to an Amazon card).

I remember I went through some automated process which asked me for some account details, and after a few hours (fewer than one day, I remember as much) my account was unlocked. It was scary!

What I remember puzzled me is that it was only two low-value cards. I would have understood the system being triggered by a lot of low value transactions, or a few large ones... but these were few and low value! Go figure.

Sorry I don't remember more details. All I know is it made me scared of gift cards.