Experienced as I am in driving my car in 40°+ temperatures, I can say that the temp gauge on the S8 will rise very slightly above the 90 mark on extremely hot days (35°+), but only in traffic or at low speeds.
Moving at good pace, the speed settles and stays on 90°, even at 42° (the highest temperature I've driven in since owning the S8 - some mild summers recently
).
With the climate control on automatic (ie. Aircon on), the aux fan runs at slow speed constantly (nearly inaudible). It will turn off on Econ mode and then run purely on temp settings.
The trinary switch will of course trigger higher fan speeds as the engine temperature rises, which may be the 'trigger' David is hearing at 94°.
Different markets may trigger this differently of course, though I don't see why they'd bother when you could potentially drive it to a hotter climate...