the question is does MR do a sweep of everyones acct to see what if any Codes they have used?

I would think the only time a person would have to worry is if they were asked for ID, didnt have it and when told they would indeed be charged the Rack Rate protested or made a scene. Most likely the Hotel would contact MR as they know that most likely the person will be doing so as well, to complain.

I have a few codes that I am able to use and depending upon which is cheaper or being offered will determine which I will use. On any given trip consisting of more then 1 property its very possible for me to use more then 1 code. As is the case with car rentals, I have found that renting in different cities during the same week that different codes will produce a less expensive rental per city, could be AAA or Costco or Sams etc etc will not work or 1 will be cheaper then the next. So I will rent AAA here and say Costco there