Well...I'm not sure how hidden it is. We know that they are Apple surveilance devices, and we know that Apple will roll over if the Feds ask them to.
That said, I'd be surprised if there weren't zero-day exploits that haven't yet been made public. OTOH, the same is true for EVERY smart phone.
We've also be informed that the NSA records 80% of all voice conversations. (True? False? No way to check.) This plausibly means that they have all cell phone towers bugged. So they probably rarely need to bother Apple for the information.
Siri clearly requires that the phone know where you are to properly understand you. (Also to communicate with you.)
So whether they were intentionally designed for the purpose of being a surveilance device or not (I lean towards not) the capabilities are there. It has also been reported that the microphones and cameras can be remotely activated without signal to the user. Bug or feature? Or did it start out as a bug, but has not been documented?
Whatever, what Apple has been accused of seems blatantly true. But perhaps a result of feature creep than of malign intentions.