It was the Der Spiegel stories that started this, although apparently UEFA knew about Fordham ten years ago and discussed it with us then, 3 years before Der Spiegel published their articles. And if Fordham was meant to be a secret, it was a poorly-concealed one as it was in full view on the Companies House website.
My view has always been that if UEFA thought we were hiding player payments from them, we'd have known about it, given how close it was after the 2014 settlement. But we know that Fordham was a rather contrived device to sell IP in order to generate a one-off increase revenue in the 2012/13 FY, not a device to hide expenses.
My guess would be that UEFA saw player payments coming from a third-party and queried this. I believe the Fordham payments carried on for another couple of years before the scheme was closed down so there couldn't have been too much of an issue with them.