It's not hidden at all. Financial blogs very accessible to laymen like Matt Levine's Money Stuff have talked about this structure months ago. If you are an investor and surprised by this news you weren't sufficiently prepared and shouldn't have been investing in the first place.
I'm with you inasmuch as the frothiness is, while massaged a bit, sufficiently perceptible for emptors to caveat.
My concern is that the subprime mortgages were not perceptible to even savvy fund managers, and the same thing could happen again; and also, even if that insidious risk-poisoning of institutional funds does not occur, them a collapsing bubble still fucks us all into recession.
What's the purpose of keeping it off the balance sheet if not to hide it?