Here's a link, from this board, on coolant sensor failure:
http://www.xoutpost.com/x5-e53-forum/...coolant+sensor
And, I rec'd dragging your X back to Stlr, as I recall yours is
"CPO"; no idea if they cover that item these days, under CPO.
The ambient temp should have no effect...something is amiss with the sensor or some connected glitch.
My 5 yr old X has never shown coolant low, though it does consume some coolant with regularity.
GL,mD