I wrote a post about it, at least how I suspect it happened. Feel free to share it, if that was how they did it then a lot of other people would most likely have had their gift cards stolen too.
2 days ago when MakerWorld was down, or a couple hours before it went down and probably the reason they took it down, I could see other peoples transaction history. I could not see my own page 1 of the transaction history, nor my redeem history. I assume someone else could see both my page of transaction history and redeem history. But I dunno, somehow they did it. Hopefully it will be an easy fix for the support, just check the code and whoever used them, match against my details and will be quite easy to see its not me, and maybe call the cops on that dude. Literally stealing…