I have never seen any mention of credentials required nor have I ever been asked for any.
However, in looking at the HGI (DEN), it clearly states "Credentials required at check-in" in the rate details, where normally it just states HHonors number required. Doing a spot check of some of my other go-to properties where I normally use MVP, none of them had that in the rate details. Interesting. Has anybody recently stayed at the DEN HGI on an MVP rate and run into issues?