When RR reads a temperature sensor, it keeps that number as the current temperature until it gets a new reading from it. So, if you plug the temperature sensor, it will change from "N/A" to the current number, when you unplug it, it will stay at that value until a new temperature value comes in OR until you restart RR or unplug the whole temperature sensor board altogether for at least 10 seconds (so RR resets the COM port connection).
All I'm saying is that unless you read the temperature sensor yourself (plugin/script/application), you won't have the ability to make such detailed control when a temperature stops being transmitted. The built-in code in RR is optimized to display temperatures and nothing else. Either you figure out a way to show specific values for the temperatures and then compare those values in the skin to determine the state of something outside OR you write your own code (plugin/app) to check for those situations of the sensor presence -- and then you can just send to RR the 2 temperatures you want to display in it.
If you have a free serial port, I'd suggest you look into this thread:
You can definitely do things based on the indicator states provided by the 3 inputs (standard) on any serial port.