Update on the rMBP10,1 fan activity. Installed 14.0 + 1.0.1 yesterday and, as we discussed, the fans were going immediately after first login, as expected. Went to bed around 9PM, no fans at that point.
I was unable to sleep and checked the rMBP at around 4AM. Fans were going noticeably fast. Checked Activity Monitor, no explanation there; CPU & Disk both nominal.
Left it running and went back to bed. Got up around 9AM, fan activity had stopped and is still silent after an hour or so of use.
My current SWAG / theory is Apple have built-in "smarts" into some aspects of the post install workflow to only run when 99%+ of users would be asleep and the m/c otherwise idle. That would be easy enough for Apple to do and a perfectly logical move. But it's just a guess on my part.