One time I was speaking to a rep. at the Platinum line and I asked about this specifically--it never happened to me, but was curious. I was told that your res. number will maintain the history of the cancellation including how it was cancelled and the cancellation number. Should have asked for those--which would have been either made up or non-existent. PLUS, ALL cancellations and reservations on my Marriott account are emailed (no matter how they are cancelled), so I would know they were lying. They OP probably was in the same situation. I wonder if that didnt happen if the reservation was still active and could have been checked on-line.

(Ok OP was working with a travel agent, but I guess they would still have the Marriott conf num.)