Oh hey, it's true. Chapter 526.
Guess it was easy to hide since he wasn't stripped like the other prisoners. A chapter before they note how the boiling bath conveniently sterilizes the cloths as well, so there was no need to change him into jailwear just for a week or so (same with Jinbe).
He also checks it in chapters 539 and 540.
You and Decide make a good point, Ace is wearing his own clothes so it wouldn't have been hard for him to have kept his vivre card hidden. Especially since the prison guards probably wouldn't have bothered to look for it. It wasn't a secret that Ace was being kept in Impel Down so why would the prison guards care if people could also tell that using a vivre card.