That may be the connector for the trim piece that has ‘premium‘ ambient lighting capability.
The original feature list for the model S talked about “ambient lighting all around” or words similar to that, but references to ambient lighting were removed closer to the release of the car. Renders of the interior still contain a light effect across the aluminium strip.
I guess they removed the lit trim due to supplier issues or quality issues in the trim pieces. Removing the loom for those connectors would cost too much or they may have a plan to introduce ambient lighting once they have perfected the design or resolved supply.
I for one hope they do introduce it in later production models. It adds to the ‘ambience’ of the cabin.