Thermopile output voltage range
The output is 0mV when the sensed temperature is the same as the package ambient.

The sensor has a thermistor in the package to detect the package ambient temperature.

It's a thermopile method. This means it's detecting the difference in temperature between the package ambient at the 'back' of the pile, and the temperature of the 'front' of the pile that's in IR 'contact' with the item being sensed. If the item is cold, the front of the pile 'cools off' and drops below ambient, giving a negative output voltage.

You'll need to check whether the scale, degrees per mV, changes at different ambient temperatures. It's not immediately obvious to me whether it should or not. Perhaps that's the responsivity tempco of typically -0.11%/C. I notice the responsivity is scaled in mV/W and not mV/target_temperature. This is as it should be, given the 4th power law for power emitted versus body temperature.