I just did a search for auxiliary power socket and found this very old thread. My 09 Touring came with one. It's $23 Triumph part noted in the first post on this thread. I noticed that a BMW R1200C I recently test rode had the same plug.
The guy who had the Beemer said he had an adapter plug for his so he could hook up his battery tender and that they are readily available for about $8. So in searching for them, what do I refer to this type of plug? Is it a European style plug? I found something called a Hela adapter on Amazon but I'm not sure if that's it. I'd never seen a power socket like this until I bought my Touring. The Beemer was the second.