I, too, got the Platinum renewal message on my Marriott account page (no email) although I was 2 nights short. FYI. I will actually be getting those 2 nights on a weekend holiday trip to Philly. But what was really odd to me is that after reading this thread, I logged in again today to check and my number of nights stayed is now showing 75! Even though no activity is showing on my account to get me from 73 to 75.

In any case, since I know I will be hitting at least my 75 nights (and I am expecting CC charges to push me a bit higher than 75) I am not stressing over "phantom" nights and/or messages appearing.