I just looked at my 2015 P85D and mine's not connected to anything although it does have a dummy plug end that it's connected into, kinda like a cap cover to protect it from dust and debris.
I does not connect to anything at all. It's just dangling there with the dummy plug cover on it. I only got 18k miles on my P85D so I'm guessing it's been like that since new?
I say just tape it up like the one I circled in your pic (that one isn't connected to anything either and it's the same on mine too):
View attachment 976301
Maybe this is part of Tesla's future plans to put something there that it will eventually connect into?