Not knowing exactly how Starwood's PMS system operates, there could be numerous plausible explanations for why that note was placed on your reservation.

The first explanation that comes to my mind, and is quite likely, is that you were upgraded or given an amenity on a previous stay and they flagged that reservation with "DNMV". The agent or system accidentally saved the code/flag to your guest profile, which automatically preflags every subsequent reservation with the same code/flag, even if it will not apply anymore.

Or perhaps you've been unruly to staff in the past and somebody wants to make sure you're not upgraded...I like the first explanation better.