Guess what (expected)
My F20 just delivered also has the rust under the seats
I am informed by BMW service here that the seat frames are an alloy and that the rust look is only a reaction from a cleaner used when they put the seat together. It will not effect the metal.
My Dealer is organising for the seats to be removed, cleaned and coated with a protective coating whilst I am away on my next trip. They will also do an inspection behind the dash vents.