Weird that it's >256. Guess they don't use 8-bit integers...or proper error-trapping.

(Sure it's great that it might be able to read temps that high, but if that was the outdoor temperature, I don't think anyone would give a damn about reading the sign.

)
The temp sensor we use here at work puts out a linear 0-2.5V signal, which correlates to -40°F - 126.5°F. An ADC on a microcontroller reads the value and translates it to a temperature value, which is then sent to a display controller.
If there's no sensor hooked to the input, you can get weird values when the ADC reads the value of any noise on the input pin.