In MR I am a PLT Premier and to MVCI I am PLT Plus. When I check-in at a Hotel it pulls from my rewards record and when I check-in at a timeshare it pulls from my vacationclub profile only if I booked it from the vacationclub account.
So it pulls only from the source marketing channel that is used to make the reservation. For example, If I were to book at a MVCI property from Marriott.com, then my MR Elite level would be displayed and they would never even know that I am an owner. I have confirmed this on many printouts of my folio.
Now if someone lands a reservation at a MVCI property with only MR profile and a MOD booking code, they have no way of verifying ownership from that record. They would have to ask for some sort of ID from the customer.
I can bet that 99% of MVCI owners can't prove this since they have not issued new ID cards in years. I still have mine from when I joined. Never recall getting any new ones either. And how many MVCI folks that do have the old cards remember to take it with them on vacation? I bet not many.
In the end, it’s only a booking code and like many have said in the past it depends on the property and check-in staff to be observant enough to notice the discrepancy.
I also rarely see anyone being challenged at Check-in to prove affiliations. It's something difficult and time consuming to verify. The front desk staff have been trained to expedite the check-in process.