Untrue. You can sniff outgoing connections and reverse engineer whatever you’d like (I sure have and am glad to have a full understanding of everything going out).
Time is another dimension here, you'd have to indefinitely monitor the device to make sure there's nothing sent out. There's also the possibility of stenography and remote-activation.
> (I sure have and am glad to have a full understanding of everything going out).
There is a finite amount of code on the system. That code would also include any functions related to stenography or remote activation and could be easily called out. However, do not exist in iOS. Anything can happen in the future, and those future versions can also be examined to find out if any such functionality was added.
The first part of my sentence only applies if you think you can/have audit(ed) every line of code running on your iOS device. If anything you're the one saying it can be done by you without any substance.
> There is a finite amount of code on the system. That code would also include any functions related to stenography or remote activation and could be easily called out.
Hint: You do not need to manually, personally, audit every single line of code to discover the use of such functionality.
