Do you mean secretive about how it works or do you mean secretive about its existence?

There is a driver for it in the Linux kernel source tree.

Looks like it came in maybe around v3.9-rc1?

http://elixir.free-electrons.com/linux/v3.9-rc1/source/drive...

Did any Linux users question what this was at that time?

Is this driver part of the "default" Linux kernel configs?

What if the user compiles their kernel without this driver?

Would that change what could or could not be done by someone accessing the "ME" remotely?