Just thought I'd post an update on my 2014 981S now with a bit over 82,000 km = 51000 mi. on it.
Last Fall, the voltage, while driving started to climb on a 200 mile drive and eventually sat at 14.9 - 15.0 V under most driving conditions for the next roughly 600 km (400 mi) which included the winter hibernation with a battery maintainer attached.
This month when I got the car out, I had the same readings on the first drive of about 60 mi. Then last weekend, we took a drive to Niagara On The Lake. Shortly into the drive, the Voltage gauge was all over the map for a few minutes, then settled in and gradually started dropping down to 13.8 V. Ever since then (about 300 km = 180 mi.), every time I've driven it, the voltage has been between 13.8 and 14.0 V, except when I put it into Sports mode. So it seems to be back to "normal".
Makes me wonder if the battery is run through some type of "super charge" mode around the 50,000 mi. or 6 years of use.