The low salt will come on whenever the conductivity of the water is reduced. This can come from a low salt level, but can also come from scale buildup on the plates. Even cells that auto-reverse polarity still need to be cleaned, just not as often.

Lower temperatures also lower conductivity, but many salt cells have temperature compensation for their sensor readings, though they will generally shut off when the temperature gets too low since the conductivity will be too low (from the low temperature) even when the salt level itself is OK.