I've run into the same thing: when checking references on what appeared to be a very good candidate, I asked, "would you hire them again?" and they said no. I pressed for details and they referred me to the legal department. The person there just read a bland statement confirming the dates of employment and the position but nothing else. Needless to say, I didn't make an offer to the person.
I think asking questions like "would you hire them again?" or "why the person left your firm?" will often uncover important information. Even if the person is not supposed to give you details, sometimes if you ask in the right way they'll be more forthcoming. And even if they won't, you can sometimes read between the lines. After all, you can never be completely sure about a candidate and if you uncover some red flags then that is probably a reason to consider someone else.