My 2001 156 2.5 Veloce recently overheated and lost pretty much all the water and coolant. I refilled with a small amount of water (approx 2 litres) and drove again before the temp gauge went into the red again. I stopped and released the expansion tank cap and the water bubbled out.
Next I tried refilling fully with water and coolant and it hasn't lost any fluid over the last 200 miles. I can't really understand this.
Having said that, I have noticed the temp guage has tended to wander beyond the 90 halfway point recently.