Got in the 2004 XK8 yesterday, during 90 degree weather here in LA. Jag has tinting and window shade, so interior was cool. But for a few seconds the outside temp showed on the Compass area of rear view mirror.
How and why did that happen, and how to control it, if I want.

 

I've never seen an outside temp display on an XK8 in the mirror? Outside temp is displayed on my 05 by holding down the climate control display button till it beeps. Then it stays on until the car is shut off. I think you saw the display segments all display for a second or 2 just after a restart. To my knowledge, there is no sensor wired to your mirror for temp display.