Followup.

I went through the tedious process of calling Hyatt Gift Card Customer Service, and establishing the redemption date of each one.

One had been redeemed back in 2013! All had the same prefix, only the last 4 digits varied. So yes I see, terribly ripe for abuse.

Cardcash first level response, says seller(s) claim I redeemed them all at once which is provably false. I just shot back with specific dates and a copy of my hotel bill.

Well at least I know now the prefix and PIN to avoid.