So it turned out to be a broken pad wear sensor wire.
 

Looks like it corroded where the wire joins the connector - on the pad side, fortunately. Can I just short the circuit to make the warning light go off?

I am surprised there was no stored code in any control module for this. I did a VAG-COM full scan and nothing turned up.

Yup. That's what you would do if you bought pads sans sensors. Stick a wire in there, and tape it.

One of our thoughts exactly. Glad it was simple.
 

There is no code, because as far as the system goes, everything is working correctly.

It logs every time the anit-theft gets triggered, and that is also 'working perfectly'.
 

Just seems odd, since there are so many controllers... I figured if it shows in the MFD, it would be logged. Oh well, such are the mysteries of a modern luxo-mobile.