some motherboards have temp sensors built in. If you have a manual see if it has an external plug, and what type of sensor it supports. There are 2 types. 3wire, and 2 wire versions.
The basis for the monitoring is based on resistance. Resistance increases with temperature on these thermistors. When there is nothing plugged in, mathematically you could call it infinite resistance. An open circuit. The software can only report in a specified range and that is why its reporting the extremely high temperatures. If you set the max temperature limit on a sensor to say 250C and unplugged it, it would go to that max value as well.
hope that helps.