I don't know what happened. I didn't get to see it, the Ford dealer did.
I do have a SWAG*. In that part of the country overheating transmissions are VERY common. They see one that lost the fluid and they think it overheated. From your description I think something happened to one of the cooler lines and that leaked out your fluid. Sometimes when that happens all you need to do is fix the leak and refill it and you're fine. Sometimes when that happens you need a new transmission.
*SWAG = Silly Wild ___ Guess
Former Ford Automatic Transmission Engineer 1988 - 2007
Nobody has made MERCON since 2006. MERCON V replaced MERCON. Get over it.