Soft 404
The ice part of the mixture is always below zero, while the water is always above zero, so I would assume the temperature of the probe depends on if it is touching ice and the gradient in the water around the rest of the probe surface. Maybe if you have slush (snow in water?) and you create a tiny pocket of water around the probe you could get just above zero, although impurities in the water could allow it to go below zero. So of course you need distilled water at sea level. 