Well, it's entirely possible there's a firmware layer below the OS doing some nefarious things or opening backdoors. I read something a long time ago about the wireless or baseband chips in Samsung phones being shown to be active even when turned off through an IR camera. A lot of people have expressed concern about and discovered vulnerabilities in the black-box baseband processors.

Microphone and camera data sounds like a bit of a stretch to me. The data storage and network throughput required to do such a thing on a large scale would be astronomical, not to mention it would be easily exposed. I think the well-known monitoring of call records and text messages and ubiquity of cloud storage for photos combined with ease of obtaining warrants and buying information more than suffices for government or large corp use, and if they do monitor mics and cameras that would be using a targeted approach.

You could always try buying a mobile base station and setting up a lab to analyze everything your phone is transmitting. I'm sure people have done similar experiments before.