so it's not hidden. I also know a few people who've worked on the kernel, and they assure me (yes I do trust them) that no such code is in the kernel.

Mind you, it's possible for some of the corporate variants to add extra code in to allow this, but I've not heard of a case where they do. Some do have availability of remote access available as applications that operate outside the kernel, and have to be specially selected for loading at installation, or later.